Small update: I have been able to run it for a total of 7 hours (2 + 2 + 3)
without a hang on the latest drm-next-4.4-wip. Tried the latest drm-fixes-4.3
and it hung after about 30 minutes, so I definitely think there's something in
4.4 that fixes it. I will do a bisect to see if I can figure out what, more
exactly, is the fix.