drm/i915: Use intel_de_rmw() for DBUF_POWER_REQUEST

Use intel_de_rmw() instead of hand rolling it.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210416171011.19012-6-ville.syrjala@linux.intel.com


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