drm/i915/gvt: fix gvt debugfs destroy

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

drm/i915/gvt: fix gvt debugfs destroy

When gvt debug fs is destroyed, need to have a sane check if drm
minor's debugfs root is still available or not, otherwise in case like
device remove through unbinding, drm minor's debugfs directory has
already been removed, then intel_gvt_debugfs_clean() would act upon
dangling pointer like below oops.

i915 0000:00:02.0: Direct firmware load for i915/gvt/vid_0x8086_did_0x1926_rid_0x0a.golden_hw_state failed with error -2
i915 0000:00:02.0: MDEV: Registered
Console: switching to colour dummy device 80x25
i915 0000:00:02.0: MDEV: Unregistering
BUG: kernel NULL pointer dereference, address: 00000000000000a0
PGD 0 P4D 0
Oops: 0002 [#1] PREEMPT SMP PTI
CPU: 2 PID: 2486 Comm: gfx-unbind.sh Tainted: G I 6.1.0-rc8+ #15
Hardware name: Dell Inc. XPS 13 9350/0JXC1H, BIOS 1.13.0 02/10/2020
RIP: 0010:down_write+0x1f/0x90
Code: 1d ff ff 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 53 48 89 fb e8 62 c0 ff ff bf 01 00 00 00 e8 28 5e 31 ff 31 c0 ba 01 00 00 00 <f0> 48 0f b1 13 75 33 65 48 8b 04 25 c0 bd 01 00 48 89 43 08 bf 01
RSP: 0018:ffff9eb3036ffcc8 EFLAGS: 00010246
RAX: 0000000000000000 RBX: 00000000000000a0 RCX: ffffff8100000000
RDX: 0000000000000001 RSI: 0000000000000064 RDI: ffffffffa48787a8
RBP: ffff9eb3036ffd30 R08: ffffeb1fc45a0608 R09: ffffeb1fc45a05c0
R10: 0000000000000002 R11: 0000000000000000 R12: 0000000000000000
R13: ffff91acc33fa328 R14: ffff91acc033f080 R15: ffff91acced533e0
FS: 00007f6947bba740(0000) GS:ffff91ae36d00000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00000000000000a0 CR3: 00000001133a2002 CR4: 00000000003706e0
Call Trace:
<TASK>
simple_recursive_removal+0x9f/0x2a0
? start_creating.part.0+0x120/0x120
? _raw_spin_lock+0x13/0x40
debugfs_remove+0x40/0x60
intel_gvt_debugfs_clean+0x15/0x30 [kvmgt]
intel_gvt_clean_device+0x49/0xe0 [kvmgt]
intel_gvt_driver_remove+0x2f/0xb0
i915_driver_remove+0xa4/0xf0
i915_pci_remove+0x1a/0x30
pci_device_remove+0x33/0xa0
device_release_driver_internal+0x1b2/0x230
unbind_store+0xe0/0x110
kernfs_fop_write_iter+0x11b/0x1f0
vfs_write+0x203/0x3d0
ksys_write+0x63/0xe0
do_syscall_64+0x37/0x90
entry_SYSCALL_64_after_hwframe+0x63/0xcd
RIP: 0033:0x7f6947cb5190
Code: 40 00 48 8b 15 71 9c 0d 00 f7 d8 64 89 02 48 c7 c0 ff ff ff ff eb b7 0f 1f 00 80 3d 51 24 0e 00 00 74 17 b8 01 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 58 c3 0f 1f 80 00 00 00 00 48 83 ec 28 48 89
RSP: 002b:00007ffcbac45a28 EFLAGS: 00000202 ORIG_RAX: 0000000000000001
RAX: ffffffffffffffda RBX: 000000000000000d RCX: 00007f6947cb5190
RDX: 000000000000000d RSI: 0000555e35c866a0 RDI: 0000000000000001
RBP: 0000555e35c866a0 R08: 0000000000000002 R09: 0000555e358cb97c
R10: 0000000000000000 R11: 0000000000000202 R12: 0000000000000001
R13: 000000000000000d R14: 0000000000000000 R15: 0000555e358cb8e0
</TASK>
Modules linked in: kvmgt
CR2: 00000000000000a0
---[ end trace 0000000000000000 ]---