I have the same issue with my lenovo ideapad s405. It came with windows 8 and all was ok. Problems started with windows 8.1, brightness buttons didn't work anymore, i tried to update drivers but didn't worked.
Lenovo released a fix for windows 10 and finally brightness worked again. Unfortunately a windows 10 update have bring back the brightness issue.
Now to use my pc i disinstalled amd driver and i'm using a general vga one.
sorry for my bad english