drm/i915/pps: use intel_de_rmw() for panel unlock

Use rmw where needed.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Reviewed-by: Andrzej Hajda <andrzej.hajda@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230417085742.793379-1-jani.nikula@intel.com