drm/i915: Extract broadwell_calc_cdclk()

Try to reduce the amount of duplicated cdclk magic numbers by
moving the max_pixclk->cdclk conversion into a helper.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1462995892-32416-3-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>