acer-wmi: Respect framebuffer blanking in backlight

If the framebuffer has requested blanking, turn the backlight down. Also
offer the user the option to do this.

Reported-by: Michal Pecio <michal.pecio@gmail.com>
Signed-off-by: Carlos Corbacho <carlos@strangeworlds.co.uk>
Signed-off-by: Len Brown <len.brown@intel.com>
Signed-off-by: Andi Kleen <ak@linux.intel.com>