drm/amd/pm: retire dimgrey_cavefish hardcode for the use of soft PPTable

The PPTable provided by VBIOS can be used.

Signed-off-by: Tao Zhou <tao.zhou1@amd.com>
Reviewed-by: Jiansong Chen <Jiansong.Chen@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>