drm/i915: Use intel_de_rmw() in skl cdclk programming

Replace the hand rolled rmw sequences with intel_de_rmw().

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210430153444.29270-4-ville.syrjala@linux.intel.com


Reviewed-by: Jani Nikula <jani.nikula@intel.com>