drm/amdgpu: fix another potential cause of VM faults

The root PD can be evicted directly after allocating it, just validate
it on first use.

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