drm/amd/powerplay: implement functions of amd_powerplay_func

This is the common interface for interacting with the powerplay
module.

v2: squash in fixes

Signed-off-by: Rex Zhu <Rex.Zhu@amd.com>
Acked-by: Jammy Zhou <Jammy.Zhou@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>