drm/i915/de: return the old register value from intel_de_rmw()

A similar thing was added in intel_uncore_rmw(). Make it available for
display too.

Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Andrzej Hajda <andrzej.hajda@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/b82cb29e8ece63e68499307f9e3e83139e590d23.1670433372.git.jani.nikula@intel.com