ACPI: video: _DOS=0 by default to prevent hotkey hang

In the past, the Linux/ACPI video driver invoked _DOS
(Display Output Switch) with the parameter 1
to tell the BIOS to switch the video output display for us.

But this conflicts with Linux native graphics drivers,
and can cause all sorts of issues, including hanging the system.

http://bugzilla.kernel.org/show_bug.cgi?id=6001

Here we change the Linux default to evaluate _DOS=0,
which tells the BIOS to simply send us a hotkey event
and not touch the graphics hardware.

The acpi video driver sends the display switch hotkey
event up through the intput layer, and X can interpret
that and use its native graphics driver to switch the display.

For the case where Linux has no native graphics driver running,
or the graphics driver doesn't know how to switch video and
the BIOS (safely) does, the previous behaviour can be restored with:

# echo 1 > /proc/acpi/video/*/DOS

Signed-off-by: Zhang Rui <rui.zhang@intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>