drm/i915: Make 308 and 671 MHz cdclks more accurate on SKL

drm/i915: Make 308 and 671 MHz cdclks more accurate on SKL

The SKL 308.57 MHz cdclk is probably 8640/28 = ~308.571 Mhz.
Similartly the 617.14 MHz cdclk is probably 8640/14 = ~617.143 MHz.
Let's use the slightly more accurate numbers. Potentially we might
change to computing all of these based on dividers, but let's
stick to the current theme for now..

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1463172100-24715-13-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Imre Deak <imre.deak@intel.com>