drm/amdgpu: cleanup amdgpu_cs_list_validate

No need to actually check the current placement. Just use the allowed domains
when the threshold is reached.

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