drm/i915: Read out hrawclk from CCK on vlv/chv

Currently we assume that hrawclk is 200MHz on VLV/CHV. That should
be true always, but just to avoid such asumptions we can read out the
actual frequency from CCK.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1456932138-14004-5-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Imre Deak <imre.deak@intel.com>