acpi_video_backlight_support() is supposed to be called by other (vendor
specific) firmware backlight controls, not by native / raw backlight controls
like nv_backlight.

Userspace will normally prefer firmware interfaces over raw interfaces, so
if acpi_video backlight support is present it will use that even if
nv_backlight is registered as well.

Except when video.use_native_backlight is present on the kernel cmdline
(or enabled through a dmi based quirk). As the name indicates the goal here
is to make only the raw interface available to userspace so that it will use
that (it only does this when it sees a win8 compliant bios).

This is done by:
1) Not registering any acpi_video# backlight devices; and
2) Making acpi_video_backlight_support() return true so that other firmware
drivers, ie acer_wmi, thinkpad_acpi, dell_laptop, etc. Don't register their
own vender specific interfaces.

Currently nouveau breaks this setup, as when acpi_video_backlight_support()
returns true, it does not register itself, resulting in no backlight control
at all.

This is esp. going to be a problem with 3.16 which will default to
video.use_native_backlight=1, and thus nouveau based laptops with a win8 bios
will get no backlight control at all.

This also likely explains why the previous attempt to make
video.use_native_backlight=1 the default was not a success, as without this
patch having a default of video.use_native_backlight=1 will cause regressions.

Note this effectively reverts commit 5bead799 (drm/nouveau: don't
expose backlight control when available through ACPI).

References: https://bugzilla.redhat.com/show_bug.cgi?id=1093171Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>