Since the /sys/device/backlight never turned up we face an issue with
disambiguating the backlight on multi-GPU devices. Both intel_backlight
and nv_backlight are presented are raw interfaces, and on modern systems
the ACPI interface is defunct, so we need a way to distinguish them. So,
we fallback to our priority table of known interfaces and rank them
accordingly, first by class (platform, native, raw) and then by priority.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>