Comment 23 for bug 1789071

One more thing. The corrected version is actually in proposed:
https://launchpad.net/ubuntu/+source/xserver-xorg-video-ati/1:18.0.1-2

I do not recommend enabling proposed in general, especially for
something so crucial to the whole system. However, since it seems you're
doing testing and not on a production system, it might be worthwhile to
enable proposed and get the likely solution:
https://wiki.ubuntu.com/Testing/EnableProposed