Comment 22 for bug 1381625

There are some scenarios make turning off brightness not desirable, e.g. an AIO user changes brightness to the lowest level in unity-control-center, he can't change it back if his keyboard does not have brightness hotkey.

Since we can't ensure that all graphics driver have the same mechanism and each one of them does not change, I think it's a better choice to let userspace program controls a consistent policy.