ACPI video: no warning message if "acpi_backlight=vendor" is used

AML code always sends notifications to ACPI video device,
even if we disable the ACPI backlight control by using
boot option "acpi_backlight=vendor".

In this case we should not print any warning message.
http://bugzilla.kernel.org/show_bug.cgi?id=13671#c14Sigend-off-by: Zhang Rui <rui.zhang@intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>

drivers/acpi/video.c

@@ -1979,6 +1979,10 @@ acpi_video_switch_brightness(struct acpi_video_device *device, int event)
 	unsigned long long level_current, level_next;
 	int result = -EINVAL;
 
+	/* no warning message if acpi_backlight=vendor is used */
+	if (!acpi_video_backlight_support())
+		return 0;
+
 	if (!device->brightness)
 		goto out;