https://bugs.freedesktop.org/show_bug.cgi?id=109006
--- Comment #1 from Robert Straw hime@is.fucking.moe --- I was going to attempt to bisect this tonight: however on a lark I decided to try 4.20-rc6 as it is nearing completion. I am pleased to report that I am *not* seeing this issue on 4.20-rc6. I suspect that the kernel commit a81a7c9 which loads polaris11_k_mc.bin (which I got from here[1]) for my particular card may have fixed the issue.
[1]: https://people.freedesktop.org/~agd5f/radeon_ucode/