acpi-video-detect: Unregister acpi_video backlight when dmi quirks are added

Make acpi_video_set_dmi_backlight_type() call
acpi_video_unregister_backlight() when the new dmi quirk results in
the desired backlight interface being of a type other then
acpi_backlight_video.

This avoid the need for the second if in the following construction
which is currently found in many platform/x86 drivers:

	if (prefer_vendor_quirk)
		acpi_video_dmi_promote_vendor();

	if (!acpi_video_backlight_support())
		acpi_video_unregister_backlight()

This second if-block will be removed from the platform drivers as part
of their conversion to the new backlight interface selection API.
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Darren Hart <dvhart@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/acpi/video_detect.c

@@ -183,6 +183,9 @@ EXPORT_SYMBOL(acpi_video_get_backlight_type);
 void acpi_video_set_dmi_backlight_type(enum acpi_backlight_type type)
 {
 	acpi_backlight_dmi = type;
+	/* Remove acpi-video backlight interface if it is no longer desired */
+	if (acpi_video_get_backlight_type() != acpi_backlight_video)
+		acpi_video_unregister_backlight();
 }
 EXPORT_SYMBOL(acpi_video_set_dmi_backlight_type);