Comment # 6 on bug 97857 from Michel Dänzer

I'm seeing this as well (on mobile Kaveri), but I think it's more likely a
kernel driver issue. E.g. I only noticed this a few months ago, but I can
reproduce it with xf86-video-ati 7.6.1, which was released almost a year ago.

Can you try older kernels? If it doesn't happen with an older kernel, can you
narrow it down to at least the kernel version which introduced it, or ideally
bisect the Git commit which did?