ACPI: video: Move Xiaomi Mi Pad 2 quirk to its own section

The Xiaomi Mi Pad 2 is currently listed under the:
"Models which should use the vendor backlight interface,
because of broken ACPI video backlight control." section.

But this is not 100% correct. The Xiaomi Mi Pad 2 is one of a set of
x86 tablets which shipped with Android as factory OS. These tablets
have a TI LP8557 backlight controller with its PWM input _not_
connected to the PMIC or chipset (LPSS) PWM output.

Instead the backlight can be controlled by configuring the LP8557
for direct control through its brightness I2C register and then
using the lp855x driver.

This setup means that neither i915's native or acpi_video backlight
control works, so a "vendor" quirk is added for these tablets to
disable both the native and acpi_video backlight devices, but these
devices do not use vendor control in the typical meaning of
vendor specific SMBIOS or ACPI calls being used.

This patch is a preparation patch for adding "vendor" quirks
for a couple more such tablet models.

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