ACPI: video: use platform backlight driver on Xiaomi Mi Pad 2

The Xiaomi Mi Pad 2 is a Cherry Trail based x86 tablet which does not
use the i915's driver backlight control support:
 i915 0000:00:02.0: [drm:intel_panel_setup_backlight [i915]] no backlight present per VBT

Like all Cherry Trail devices the ACPI tables on the Xiaomi Mi Pad 2
contain a broken ACPI-video implementation which causes 6 different
acpi_video backlights to get registered when used.

The lack of the i915 driver registering a BACKLIGHT_RAW (aka native) type
backlight causes acpi_video_get_backlight_type() to pick the broken
acpi_video backlight code as the backlight driver to use.

There actually is a separate lp8556 backlight controller connected
over I2C which gets registered as a BACKLIGHT_PLATFORM (aka vendor).

Add a quirk to force acpi_video_get_backlight_type() to return
acpi_backlight_vendor, so that the broken acpi_video backlight
interfaces do not get registered.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>