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/