drm/amdgpu/pm: fix the null pointer while the smu is disabled

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

drm/amdgpu/pm: fix the null pointer while the smu is disabled

It needs to check if the pp_funcs is initialized while release the
context, otherwise it will trigger null pointer panic while the software
smu is not enabled.

[ 1109.404555] BUG: kernel NULL pointer dereference, address: 0000000000000078
[ 1109.404609] #PF: supervisor read access in kernel mode
[ 1109.404638] #PF: error_code(0x0000) - not-present page
[ 1109.404657] PGD 0 P4D 0
[ 1109.404672] Oops: 0000 [#1] PREEMPT SMP NOPTI
[ 1109.404701] CPU: 7 PID: 9150 Comm: amdgpu_test Tainted: G OEL 5.16.0-custom #1
[ 1109.404732] Hardware name: innotek GmbH VirtualBox/VirtualBox, BIOS VirtualBox 12/01/2006
[ 1109.404765] RIP: 0010:amdgpu_dpm_force_performance_level+0x1d/0x170 [amdgpu]
[ 1109.405109] Code: 5d c3 44 8b a3 f0 80 00 00 eb e5 66 90 0f 1f 44 00 00 55 48 89 e5 41 57 41 56 41 55 41 54 53 48 83 ec 08 4c 8b b7 f0 7d 00 00 <49> 83 7e 78 00 0f 84 f2 00 00 00 80 bf 87 80 00 00 00 48 89 fb 0f
[ 1109.405176] RSP: 0018:ffffaf3083ad7c20 EFLAGS: 00010282
[ 1109.405203] RAX: 0000000000000000 RBX: ffff9796b1c14600 RCX: 0000000002862007
[ 1109.405229] RDX: ffff97968591c8c0 RSI: 0000000000000001 RDI: ffff9796a3700000
[ 1109.405260] RBP: ffffaf3083ad7c50 R08: ffffffff9897de00 R09: ffff979688d9db60
[ 1109.405286] R10: 0000000000000000 R11: ffff979688d9db90 R12: 0000000000000001
[ 1109.405316] R13: ffff9796a3700000 R14: 0000000000000000 R15: ffff9796a3708fc0
[ 1109.405345] FS: 00007ff055cff180(0000) GS:ffff9796bfdc0000(0000) knlGS:0000000000000000
[ 1109.405378] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 1109.405400] CR2: 0000000000000078 CR3: 000000000a394000 CR4: 00000000000506e0
[ 1109.405434] Call Trace:
[ 1109.405445] <TASK>
[ 1109.405456] ? delete_object_full+0x1d/0x20
[ 1109.405480] amdgpu_ctx_set_stable_pstate+0x7c/0xa0 [amdgpu]
[ 1109.405698] amdgpu_ctx_fini.part.0+0xcb/0x100 [amdgpu]
[ 1109.405911] amdgpu_ctx_do_release+0x71/0x80 [amdgpu]
[ 1109.406121] amdgpu_ctx_ioctl+0x52d/0x550 [amdgpu]
[ 1109.406327] ? _raw_spin_unlock+0x1a/0x30
[ 1109.406354] ? drm_gem_handle_delete+0x81/0xb0 [drm]
[ 1109.406400] ? amdgpu_ctx_get_entity+0x2c0/0x2c0 [amdgpu]
[ 1109.406609] drm_ioctl_kernel+0xb6/0x140 [drm]