drm/amd/powerplay: move od8_setting helper function to vega20_ppt

these callback functions is only used for vega20 asic, to be compatible
other asics,need to move this code to vega20_ppt file

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>