drm/i915: s/required_vco/vco/ in skl cdclk code

The 'required' part of 'required_vco' should be obvious. Let's just call
it 'vco' for brevity.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1462995892-32416-12-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>