drm/amdgpu: fix VM flush for CPU based updates

We don't have any update fence in that case, so the need
for flushing isn't detected automatically.

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>