drm/i915/display/dsi: use intel_de_rmw if possible

The helper makes the code more compact and readable.
Signed-off-by: Andrzej Hajda <andrzej.hajda@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221219130844.2914001-1-andrzej.hajda@intel.com