drm/i915: Store max cdclk value in dev_priv

Keep the cdclk maximum supported frequency around in dev_priv so that we
can verify certain things against it before actually changing the cdclk
frequency.

For now only VLV/CHV have support changing cdclk frequency, so other
plarforms get to assume cdclk is fixed.

v2: Rebased to the latest
v3: Rebased to the latest
v4: Fix for patch style problems

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Mika Kahola <mika.kahola@intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>