https://bugs.freedesktop.org/show_bug.cgi?id=109246
--- Comment #9 from tme t00m4nyerr0rs@gmail.com --- I've tested some more kernels, all from https://kernel.ubuntu.com/~kernel-ppa/mainline/.
Setting amdgpu.dc=1 on 4.15.18 and 4.16.18 does not reproduce the problem. It does, however, reproduce on 4.17.19.
Also, I now believe my HDMI monitor may have polling, as it cycles through each input once when it no longer detects a signal.
Please let me know if there is anything else I can help with.