drm/amd/powerplay: cleanup the interfaces for powergate setting through SMU

Provided an unified entry point. And fixed the confusing that the API
usage is conflict with what the naming implies.

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>