ATM on x86 laptops where we want userspace to use the acpi_video backlight
device we often register both the GPU's native backlight device and
acpi_video's firmware acpi_video# backlight device. This relies on
userspace preferring firmware type backlight devices over native ones, but
registering 2 backlight devices for a single display really is undesirable.

On x86 laptops where the native GPU backlight device should be used,
the registering of other backlight devices is avoided by their drivers
using acpi_video_get_backlight_type() and only registering their backlight
if the return value matches their type.

acpi_video_get_backlight_type() uses
backlight_device_get_by_type(BACKLIGHT_RAW) to determine if a native
driver is available and will never return native if this returns
false. This means that the GPU's native backlight registering code
cannot just call acpi_video_get_backlight_type() to determine if it
should register its backlight, since acpi_video_get_backlight_type() will
never return native until the native backlight has already registered.

To fix this add a new internal native function parameter to
acpi_video_get_backlight_type(), which when set to true will make
acpi_video_get_backlight_type() behave as if a native backlight has
already been registered.

And add a new acpi_video_backlight_use_native() helper, which sets this
to true, for use in native GPU backlight code.

Changes in v2:
- Replace adding a native parameter to acpi_video_get_backlight_type() with
  adding a new acpi_video_backlight_use_native() helper.
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>