commit 6a3ef10b upstream.

The L521X variant of the Dell XPS15 has integrated nvidia graphics, and
backlight control does not work properly when using the native interfaces.

Link: https://bugzilla.redhat.com/show_bug.cgi?id=1163574Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>