acpi: add a way to promote/demote vendor backlight drivers

Instead of adding a big blacklist in video_detect.c to set
ACPI_VIDEO_BACKLIGHT_DMI_VENDOR correctly, let external modules
promote or demote themselves when they know the generic video
module won't work.

Currently drivers where using acpi_video_unregister() directly
but:
- That didn't respect any acpi_backlight=[video|vendor] parameter
  provided by the user.
- Any later call to acpi_video_register() would still re-load the
  generic video module (and some gpu drivers are doing that).

This patch fix those two issues.

Signed-off-by: Corentin Chary <corentin.chary@gmail.com>
Signed-off-by: Matthew Garrett <mjg@redhat.com>