drm/amdgpu: validate the VM root PD from the VM code

Preparation for following changes. This validates the root PD twice,
but the overhead of that should be minimal.

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