drm/amdgpu: Add a NULL check for freeing root PT

drm/amdgpu: Add a NULL check for freeing root PT

This patch adds a NULL check to fix this crash reported during the
freeing of root PT entry:

 BUG: unable to handle page fault for address: ffffc9002d637aa0
 #PF: supervisor write access in kernel mode
 #PF: error_code(0x0002) - not-present page
 RIP: 0010:amdgpu_vm_pt_free+0x66/0xe0 [amdgpu]
 PKRU: 55555554
 Call Trace:
 <TASK>
  amdgpu_vm_pt_free_root+0x60/0xa0 [amdgpu]
  amdgpu_vm_fini+0x2cb/0x5d0 [amdgpu]
  ? amdgpu_ctx_mgr_entity_fini+0x53/0x1c0 [amdgpu]
  amdgpu_driver_postclose_kms+0x191/0x2d0 [amdgpu]
  drm_file_free.part.0+0x1e5/0x260 [drm]

Cc: Christian König <Christian.Koenig@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Felix Kuehling <felix.kuehling@amd.com>
Cc: Rajneesh Bhardwaj <rajneesh.bhardwaj@amd.com>
Acked-by: Christian König <Christian.Koenig@amd.com>
Signed-off-by: Shashank Sharma <shashank.sharma@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>