acer-wmi: Switch to acpi_video_unregister_backlight

Switch from acpi_video_unregister(), to acpi_video_unregister_backlight(),
so that the hotkeys handler registered by acpi-video stays in place.

Since there are no mappings for the atkbd raw codes for the brightness
keys used by newer Acer models in /lib/udev/hwdb.d/60-keyboard.hwdb, and
since we map the wmi events with a code of KE_IGNORE, we rely on acpi-video
to do the hotkey handling for us.

For laptops such as the Acer Aspire 5750 which uses intel gfx this works
despite us calling acpi_video_unregister() because the following happens:

 1) acpi-video module gets loaded (as it is a dependency of acer-wmi and i915)
 2) acpi-video does NOT call acpi_video_register()
 3) acer-wmi loads (assume it loads before i915), calls
    acpi_video_dmi_promote_vendor(); which sets
    ACPI_VIDEO_BACKLIGHT_DMI_VENDOR
 4) calls acpi_video_unregister -> not registered, nop
 5) i915 loads, calls acpi_video_register
 6) acpi_video_register registers the acpi_notifier for the hotkeys,
    does NOT register a backlight device because of
    ACPI_VIDEO_BACKLIGHT_DMI_VENDOR

But on the Acer Aspire 5750G, which uses nvidia graphics the following happens:
 1) acpi-video module gets loaded (as it is a dependency of acer-wmi)
 2) acpi-video calls acpi_video_register()
 3) acpi_video_register registers the acpi_notifier for the hotkeys,
    and a backlight device
 4) acer-wmi loads, calls acpi_video_dmi_promote_vendor()
 5) calls acpi_video_unregister, this unregisters BOTH the acpi_notifier for
    the hotkeys AND the backlight device

And we end up without any handler for the brightness hotkeys. This patch fixes
this by switching over to acpi_video_unregister_backlight() which keeps the
hotkey handler in place.

References: https://bugzilla.kernel.org/show_bug.cgi?id=35622
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

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