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

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

Replace the hand rolled rmw sequences with intel_de_rmw().

Jani pointed out that intel_de_rmw() skips the write if the
value does not change. That should be totally fine here, but
let's at least acknowledge the change in behaviour in case I'm
somehow wrong...

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210430153444.29270-3-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>