drm/amdgpu/sdma4: Add placeholder for vega12 golden settings

Fill these in when we get them.

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