two things to add to this bug:
1) I've tried 4.3.0 and 4.4.0 kernels as well and the symptom is the same - strongly suggests this is a user space problem.
2) BUT system immediately hangs when starting chrome or openarena when I added /etc/modprobe.d/i915.conf with following content:
options i915 enable_rc6=1 enable_fbc=1 lvds_downclock=1 semaphores=1
I also tried enable_rc6=0 and permutations with enable_fbc=0 (or 1).
lightdm is able to throw up a login screen and I can use xterm with or without the i915.conf in place.
After removing the i915.conf, I can start chrome, but it eventually hangs on some websites. I'm assuming the content is exercising the GPU in the same way that openarena might.