drm/xe: Prevent null pointer access in xe_migrate_copy

In the Linux kernel, the following vulnerability has been resolved:

drm/xe: Prevent null pointer access in xe_migrate_copy

xe_migrate_copy designed to copy content of TTM resources. When source
resource is null, it will trigger a NULL pointer dereference in
xe_migrate_copy. To avoid this situation, update lacks source flag to
true for this case, the flag will trigger xe_migrate_clear rather than
xe_migrate_copy.

Issue trace:
<7> [317.089847] xe 0000:00:02.0: [drm:xe_migrate_copy [xe]] Pass 14,
sizes: 4194304 & 4194304
<7> [317.089945] xe 0000:00:02.0: [drm:xe_migrate_copy [xe]] Pass 15,
sizes: 4194304 & 4194304
<1> [317.128055] BUG: kernel NULL pointer dereference, address:
0000000000000010
<1> [317.128064] #PF: supervisor read access in kernel mode
<1> [317.128066] #PF: error_code(0x0000) - not-present page
<6> [317.128069] PGD 0 P4D 0
<4> [317.128071] Oops: Oops: 0000 [#1] PREEMPT SMP NOPTI
<4> [317.128074] CPU: 1 UID: 0 PID: 1440 Comm: kunit_try_catch Tainted:
G U N 6.11.0-rc7-xe #1
<4> [317.128078] Tainted: [U]=USER, [N]=TEST
<4> [317.128080] Hardware name: Intel Corporation Lunar Lake Client
Platform/LNL-M LP5 RVP1, BIOS LNLMFWI1.R00.3221.D80.2407291239 07/29/2024
<4> [317.128082] RIP: 0010:xe_migrate_copy+0x66/0x13e0 [xe]
<4> [317.128158] Code: 00 00 48 89 8d e0 fe ff ff 48 8b 40 10 4c 89 85 c8
fe ff ff 44 88 8d bd fe ff ff 65 48 8b 3c 25 28 00 00 00 48 89 7d d0 31
ff <8b> 79 10 48 89 85 a0 fe ff ff 48 8b 00 48 89 b5 d8 fe ff ff 83 ff
<4> [317.128162] RSP: 0018:ffffc9000167f9f0 EFLAGS: 00010246
<4> [317.128164] RAX: ffff8881120d8028 RBX: ffff88814d070428 RCX:
0000000000000000
<4> [317.128166] RDX: ffff88813cb99c00 RSI: 0000000004000000 RDI:
0000000000000000
<4> [317.128168] RBP: ffffc9000167fbb8 R08: ffff88814e7b1f08 R09:
0000000000000001
<4> [317.128170] R10: 0000000000000001 R11: 0000000000000001 R12:
ffff88814e7b1f08
<4> [317.128172] R13: ffff88814e7b1f08 R14: ffff88813cb99c00 R15:
0000000000000001
<4> [317.128174] FS: 0000000000000000(0000) GS:ffff88846f280000(0000)
knlGS:0000000000000000
<4> [317.128176] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
<4> [317.128178] CR2: 0000000000000010 CR3: 000000011f676004 CR4:
0000000000770ef0
<4> [317.128180] DR0: 0000000000000000 DR1: 0000000000000000 DR2:
0000000000000000
<4> [317.128182] DR3: 0000000000000000 DR6: 00000000ffff07f0 DR7:
0000000000000400
<4> [317.128184] PKRU: 55555554
<4> [317.128185] Call Trace:
<4> [317.128187] <TASK>
<4> [317.128189] ? show_regs+0x67/0x70
<4> [317.128194] ? __die_body+0x20/0x70
<4> [317.128196] ? __die+0x2b/0x40
<4> [317.128198] ? page_fault_oops+0x15f/0x4e0
<4> [317.128203] ? do_user_addr_fault+0x3fb/0x970
<4> [317.128205] ? lock_acquire+0xc7/0x2e0
<4> [317.128209] ? exc_page_fault+0x87/0x2b0
<4> [317.128212] ? asm_exc_page_fault+0x27/0x30
<4> [317.128216] ? xe_migrate_copy+0x66/0x13e0 [xe]
<4> [317.128263] ? __lock_acquire+0xb9d/0x26f0
<4> [317.128265] ? __lock_acquire+0xb9d/0x26f0
<4> [317.128267] ? sg_free_append_table+0x20/0x80
<4> [317.128271] ? lock_acquire+0xc7/0x2e0
<4> [317.128273] ? mark_held_locks+0x4d/0x80
<4> [317.128275] ? trace_hardirqs_on+0x1e/0xd0
<4> [317.128278] ? _raw_spin_unlock_irqrestore+0x31/0x60
<4> [317.128281] ? __pm_runtime_resume+0x60/0xa0
<4> [317.128284] xe_bo_move+0x682/0xc50 [xe]
<4> [317.128315] ? lock_is_held_type+0xaa/0x120
<4> [317.128318] ttm_bo_handle_move_mem+0xe5/0x1a0 [ttm]
<4> [317.128324] ttm_bo_validate+0xd1/0x1a0 [ttm]
<4> [317.128328] shrink_test_run_device+0x721/0xc10 [xe]
<4> [317.128360] ? find_held_lock+0x31/0x90
<4> [317.128363] ? lock_release+0xd1/0x2a0
<4> [317.128365] ? __pfx_kunit_generic_run_threadfn_adapter+0x10/0x10
[kunit]
<4> [317.128370] xe_bo_shrink_kunit+0x11/0x20 [xe]
<4> [317.128397] kunit_try_run_case+0x6e/0x150 [kunit]
<4> [317.128400] ? trace_hardirqs_on+0x1e/0xd0
<4> [317.128402] ? _raw_spin_unlock_irqrestore+0x31/0x60
<4> [317.128404] kunit_generic_run_threadfn_adapter+0x1e/0x40 [ku
---truncated---