drm/amdgpu: Checkpoint and Restore VRAM BOs without VA

Extend checkpoint logic to allow inclusion of VRAM BOs that
do not have a VA attached

Signed-off-by: Ramesh Errabolu <Ramesh.Errabolu@amd.com>
Reviewed-by: Felix Kuehling <felix.kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>