drm/amdgpu: partial revert VM sync changes

We still need to add the VM update fences to the root PD.

So make sure to never sync to those implicitely.

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>