drm/i915/display/power: 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: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230217111836.864959-1-andrzej.hajda@intel.com