https://bugs.freedesktop.org/show_bug.cgi?id=104639
--- Comment #4 from Christian König ckoenig.leichtzumerken@gmail.com --- Sorry, just seen that you wrote that you are using 4-15-rc8 and that should include the patch.
No idea what's going wrong here. You not by any chance could add a serial/network console and grab the last logs before the reboot?