Created attachment 114792 [details]
Xorg.0.log after failure

I have the same issue.
In fact, I've been having this issue for years, probably since I own the
hardware.

The issue happens when I turn off my screen and turn it back on, and does so
reproducibly.
It only affects DisplayPort.

My card is a Cayman, and I'm building the X stack from git.

I've attached the relevant dmesg and the Xorg.0.log directly after the issue
appeared.