drm/i915: Only restore backlight combination mode reg for ums

This was forgotten in

commit 565ee389
Author: Jani Nikula <jani.nikula@intel.com>
Date:   Wed Nov 13 12:56:29 2013 +0200

    drm/i915: do not save/restore backlight registers in KMS

Since the confusion was likely due to the duplicated definition for
this pci config register, let's unify that, too.

Cc: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>