drm/amdgpu: sync to buffer moves before VM updates

Otherwise we could update the VM page tables while the move is only scheduled.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>