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>