drm/amd/powerplay: commonize the API for retrieving current clocks

So that it can be shared between all clocks.

Signed-off-by: Evan Quan <evan.quan@amd.com>
Reviewed-by: Feifei <Xu&lt;Feifei.Xu@amd.com>
Reviewed-by: Rex Zhu <Rex.Zhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>