drm/amd/powerplay: commit get_performance_level API as DAL needed

This can suppress the error reported on driver loading. Also these
are empty APIs as Vega12/Vega20 has no performance levels.

Signed-off-by: Evan Quan <evan.quan@amd.com>
Reviewed-by: Feifei Xu <Feifei.Xu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>