ACPI: video: Add Apple GMUX brightness control detection

On Apple laptops with an Apple GMUX using this for brightness control,
should take precedence of any other brightness control methods.

Add apple-gmux detection to acpi_video_get_backlight_type() using
the already existing apple_gmux_present() helper function.

This will allow removig the (ab)use of:

	acpi_video_set_dmi_backlight_type(acpi_backlight_vendor);

Inside the apple-gmux driver.

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