drm/i915/vlv: split CCK and DDR freq usage

It's possible that the CCK clock could run at a different rate than the
DDR clock, so use the same method to get CCK as the GMBUS code does when
calculating the new CDclk divider in the VLV display code.

Reported-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>