This is a Macbook pro 8,2 with an Intel HD Graphics 3000 [8086:0116] and an AMD
Radeon HD 6750M [1002:6741].
The backlight interface in /sys/class/backlight/radeon_bl0 does not work.
Echoing to "brightness" does not change the brightness of the Monitor. There is
also an apple_gmux interface, which works though.
I've bound xbacklight to the brightness keys to control the brightness, but it
looks like it's confused about which interface it should use:
$ xbacklight -inc 5
No outputs have backlight property

When I disable the Radeon card on boot and use the Intel card, xbacklight
works.