drm/amd/pp: implement dpm_get_sclk/mclk for RV

RV implementation was missing these callbacks.  Used
to fetch the clock values for other components.

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