ACPI: video: Refactor acpi_video_get_backlight_type() a bit

Refactor acpi_video_get_backlight_type() so that the heuristics /
detection steps are stricly in order of descending precedence.

Also move the comments describing the steps to when the various steps are
actually done, to avoid the comments getting out of sync with the code.

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