Beacuse of PTI, I decided to test 4.15-rc6 on most of my test machines. And I happened to notice a radeon problem on RV250 era card with no monitor attached. 4.15-rc1 and earlier kernels did not show these errors, not kernels between rc1 and rc6 have been tested yet.
Sorry, I can not reproduce it, so it looks like random noise from an old computer. About 10 reboots later, I have not managed to reproduce this problem.