ACPI / video: no automatic brightness changes by win8-compatible firmware

Starting from win8, MS backlight control driver will set bit 2 of the
parameter of control method _DOS, to inform firmware it should not
perform any automatic brightness changes. This mostly affects hotkey
notification deliver - if we do not set this bit, on hotkey press,
firmware may choose to adjust brightness level instead of sending out
notification and doing nothing.

So this patch sets bit 2 when calling _DOS so that GUIs can show the
notification window on hotkey press.  This behavior change is only
necessary for win8 systems.

The MS document on win8 backlight control is here:
http://msdn.microsoft.com/en-US/library/windows/hardware/jj159305

References: https://bugzilla.kernel.org/show_bug.cgi?id=52951
References: https://bugzilla.kernel.org/show_bug.cgi?id=56711
Reported-by: Micael Dias <kam1kaz3@gmail.com>
Reported-by: Dan Garton <dan.garton@gmail.com>
Reported-by: Bob Ziuchkovski <bob.ziuchkovski@gmail.com>
Signed-off-by: Aaron Lu <aaron.lu@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/acpi/video.c

@@ -1539,14 +1539,20 @@ static int acpi_video_bus_put_devices(struct acpi_video_bus *video)
 
 /* acpi_video interface */
 
+/*
+ * Win8 requires setting bit2 of _DOS to let firmware know it shouldn't
+ * preform any automatic brightness change on receiving a notification.
+ */
 static int acpi_video_bus_start_devices(struct acpi_video_bus *video)
 {
-	return acpi_video_bus_DOS(video, 0, 0);
+	return acpi_video_bus_DOS(video, 0,
+				  acpi_video_backlight_quirks() ? 1 : 0);
 }
 
 static int acpi_video_bus_stop_devices(struct acpi_video_bus *video)
 {
-	return acpi_video_bus_DOS(video, 0, 1);
+	return acpi_video_bus_DOS(video, 0,
+				  acpi_video_backlight_quirks() ? 0 : 1);
 }
 
 static void acpi_video_bus_notify(struct acpi_device *device, u32 event)