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>