Ran more tests and tried to bisect mesa, but the end result is something weird that I can't figure out anymore where the bug is. Using whatever combination of kernel, llvm, mesa, or the radeon X driver on Ubuntu 16.04 seems to work, but fails on 17.04. Took an apitrace of running Faeria: https://aaltoset.kapsi.fi/tmp/Faeria.x86_64.trace.xz