BREAKING FreeBSD NEWS: AMDGPU DC works!
The 4.15 drm update apparently did not have amdgpu working yet... but I tried it anyway, since that's just an assertion failure.
Well... it does work. Even with DC (
compat.linuxkpi.dc="1")! Weston didn't work at first, but commenting out its "Disable all the planes" section that was causing a kernel warning solved the problem.
So quite possibly we have Vega support?