drm/amdgpu: Handle NULL bo->tbo.resource (again) in amdgpu_vm_bo_update

Third time's the charm, I hope?

Fixes: d311675 ("drm/ttm: rename bo->mem and make it a pointer")
Closes: https://gitlab.freedesktop.org/drm/amd/-/issues/3837
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Michel Dänzer <mdaenzer@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c

@@ -1266,10 +1266,9 @@ int amdgpu_vm_bo_update(struct amdgpu_device *adev, struct amdgpu_bo_va *bo_va,
 	 * next command submission.
 	 */
 	if (amdgpu_vm_is_bo_always_valid(vm, bo)) {
-		uint32_t mem_type = bo->tbo.resource->mem_type;
-
-		if (!(bo->preferred_domains &
-		      amdgpu_mem_type_to_domain(mem_type)))
+		if (bo->tbo.resource &&
+		    !(bo->preferred_domains &
+		      amdgpu_mem_type_to_domain(bo->tbo.resource->mem_type)))
 			amdgpu_vm_bo_evicted(&bo_va->base);
 		else
 			amdgpu_vm_bo_idle(&bo_va->base);