If we have a generic framebuffer device then there should be no
need to set up any backlight in early boot.

Also, backlight devices are sometimes handled by large drm drivers
like i915 which we don't want to include by default.

Fixes: da851903 ("hook-functions: Include modules for Chromebook ...")
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>