drm/amdgpu: fix NULL ptr dref in the VM code

The exclusive fence is of course perfectly optional here.
Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

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

@@ -1789,9 +1789,11 @@ static int amdgpu_vm_bo_update_mapping(struct amdgpu_device *adev,
 			return r;
 
 		/* Wait for any BO move to be completed */
+		if (exclusive) {
 			r = dma_fence_wait(exclusive, true);
 			if (unlikely(r))
 				return r;
+		}
 
 		params.func = amdgpu_vm_cpu_set_ptes;
 		params.pages_addr = pages_addr;