drm/i915: Extract skl_dpll0_disable()

Make thins a bit easier to read by extracting the SKL DPLL0
disable into separate functions. We already have the enable
counterpart. Down the line this will also help make the cdclk
programming on SKL, BXT, and following platforms look rather
consistent.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1462995892-32416-9-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>