asus-wmi: Disable acpi-video backlight on desktop machines

Some Asus motherboards for desktop PC-s export an acpi-video interface
advertising backlight support. Test the dmi chassis-type and tell acpi-video
to not register a backlight interface on desktops.

https://bugzilla.redhat.com/show_bug.cgi?id=1097436



Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Matthew Garrett <matthew.garrett@nebula.com>