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

The backlight enable code now has the smarts to do the right thing. Only
do backlight register save/restore in UMS.

Some VLV specific code gets dropped as UMS is not supported on VLV.

v2: Move save/restore to UMS instead of removing completely (Daniel).

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