drm/i915: Extract skl_calc_cdclk()

We have many places where we want to pick a suitable cdclk frequency for
skl based on the dotclock and lcpll vco. Split that code into a small
helper and call it from all over.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1463172100-24715-5-git-send-email-ville.syrjala@linux.intel.com


Reviewed-by: Imre Deak <imre.deak@intel.com>