Normal brightness keys which adjust the brightness one step won't work.
I faced the same issue on my dell inspiron 15 7559. I had 4 kernel versions installed and out of them two were giving this issue while the older 2 didnt.
Turns out, updating your kernel to the latest recommended kernel in Ubuntu 16.04.3 fixed this for me (after having tried tons of alternative "solutions").
To do this update, run I wish I had kept as detailed notes as you.
So I updated to 4.7.1, then 4.7.2 and finally 4.7.3.I recommend setting the option to what you really want it to do, ie " 4.6.0 is deprecated along with the rest of the 4.6.x chain.