ACPI / video: Add a quirk to force acpi-video backlight on Dell XPS L421X

Just like the Dell XPS 15 (L521X) the Dell XPS 14 (L421X) needs to use
the acpi-video backlight interface rather then the native one for backlight
control to work, add a quirk for this.

Link: https://bugzilla.redhat.com/show_bug.cgi?id=1272633


Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>