drm/amd/powerplay: drop unused code

Those code were obsoleted by new common API
smu_cmn_to_asic_specific_index().

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