drm/amdgpu: stop pipelining VM PDs/PTs moves

We are going to need this for recoverable page fault handling and it
makes shadow handling during GPU reset much more easier.

Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Junwei Zhang <Jerry.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>