Currently the video module export backlight interface to sysfs also if there is 
no _BCL ACPI method in the bios or if acpi_video_device_lcd_query_levels() fail 
for some reason. 

Because of the failure the user can't set the brightness (echo return with: 
"write error: Invalid Argument") also because the user don't know the allowed/
available brightness level. One example is e.g. a ASUS R1F Laptop which is 
correct supported by the asus-laptop module.

The kernel should not export the backlight interface if _BLC is missing because 
the interface is needless in this case. See attached patch.