drm/i915: Kill off dead code from skl_dpll0_enable()

We calculate the CDCLK_CTL value from scratch so no need to attempt
some form of RMW first.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1462995892-32416-10-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>