drm/amd/powerplay: remove upload_dpm_level function for vega20

the function upload_dpm_level is an internal function,
so remove public interface.

Signed-off-by: Kevin Wang <kevin1.wang@amd.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>