Well, since you are in Mint, you can use the Driver Manager. Just click the open source driver and apply it Then restart. That should do it, but I can't verify for sure because I don't have an AMD card. I do have one at home on a touch-screen PC with Mint 17.3 I could play with it on that one. I was able to run a bunch of games on it, but nothing as high quality as Portal 2. Its an older card
Really? How strange is that? Mine always shows either open-source (Nouvea) or Nvidia drivers. So how did you install AMD's Catalyst if the Driver Manager is blank?