ACPI / video: drop unused fields from struct acpi_video_brightness_flags

The _BCM_use_index and _BCL_use_index fields in struct
acpi_video_brightness_flags are not used, so drop them.

[rjw: Changelog]
Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>