drm/amdgpu: move shadow bo validation to VM code

Do the shadow bo validation in the VM code as
VM code knows/owns shadow BOs.

Signed-off-by: Nirmoy Das <nirmoy.das@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>