drm/amdgpu: band aid validating VM PTs

Always validating the VM PTs takes to much time. Only always validate
the per VM BOs for now.

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>