drm/amdgpu: fix invalid fence handling in amdgpu_vm_tlb_flush

CPU based update doesn't produce a fence, handle such cases properly.

Fixes: d8a3f0a0 ("drm/amdgpu: implement TLB flush fence")
Signed-off-by: Lang Yu <lang.yu@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

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

@@ -908,10 +908,12 @@ amdgpu_vm_tlb_flush(struct amdgpu_vm_update_params *params,
 {
 	struct amdgpu_vm *vm = params->vm;
 
-	if (!fence || !*fence)
+	tlb_cb->vm = vm;
+	if (!fence || !*fence) {
+		amdgpu_vm_tlb_seq_cb(NULL, &tlb_cb->cb);
 		return;
+	}
 
-	tlb_cb->vm = vm;
 	if (!dma_fence_add_callback(*fence, &tlb_cb->cb,
 				    amdgpu_vm_tlb_seq_cb)) {
 		dma_fence_put(vm->last_tlb_flush);