acpi: video: Let OS handle brightness level

Starting from win8, MS backlight control driver will set bit 2 of the
parameter of control method _DOS, to let firmware know OS will handle
backlight level and firmware should not change backlight level.

So this patch set bit 2 when calling _DOS, to make acpi video driver
work for firmwares adhere to win8 requirements.

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

Buglink:https://bugzilla.kernel.org/show_bug.cgi?id=52951
Reported-by: Micael Dias <kam1kaz3@gmail.com>
acpi: video: Add some debug statement for bug fixing

Add some debug statements so that when user reports a bug, we can easily
collect information without letting user apply some debug patch and
re-compile the kernel.

Signed-off-by: Aaron Lu <aaron.lu@intel.com>