drm/amd/powerplay: retrieve the real-time coreClock values

 - Currently, the coreClock value for min/max performance level on raven
   is hard-coded. Use the real-time value retrieved by GetGfxMinFreqLimit
   and GetGfxMaxFreqLimit PPSMC messages

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