backlight: Add backlight device (un)registration notification

Some firmware drivers, ie acpi-video want to get themselves out of the
way (in some cases) when their also is a raw backlight device available.

Due to module loading ordering being unknown, acpi-video cannot be certain
that the backlight_device_registered(BACKLIGHT_RAW) it does for this is
the final verdict wrt there being a BACKLIGHT_RAW device.

By adding notification acpi-video can listen for backlight devices showing
up after it has loaded, and unregister its backlight device if desired.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>