Hi there! Probably I chose wrong i915 platform when created bug, so, if it is, change it to the right one, please.
I'm running i7-5700HQ APU with Broadwell-U (HD 5600, afair) GPU on board. Unfortunatelly, I was unable to setup userland (Xorg) part by just installing corresponding distro package with driver with SNA enabled, so I: 1) installed xorg-driver from yesterday git master, disabled SNA and enabled UXA (both on "./configure"-time, so I didn't touch xorg.conf at all and using Xorg autodetection), added "video.use_native_backlight=1 i915.semaphores=1 drm_kms_helper.poll=N i915.enable_rc6=1 i915.enable_fbc=0 intel_idle.max_cstate=1 acpi_osi=Linux i915.enable_execlists=0" to kernel commandline, and also "export hiz=false INTEL_HIZ=0 INTEL_SEPARATE_STENCIL=0 vblank_mode=0 LIBGL_ALWAYS_SOFTWARE=1;" to /etc/profile.
Without that changes I experienced system freezes (so even SysRq-magic didn't work) in few moments after enabling compositing effects in any of my DEs (KDE5, Enlightenment). After described changes (not sure, what exactly helped) all kinda started to work fine (including compositing), but from time to time, during work I still experience system freezes (although, thankfully, not all of them are total, so, say, now I was able to switch to tty console and writing that report in lynx).  
So, I opened dmesg | less and inspected what happened. And found there suggestion to report "_new_ bug", so here I am.

P.S. Sorry, I'm not sure how lynx will handle text I just entered if I press "add attachment", so I'll include /sys/class/drm/card0/error and dmesg cutoff on next update.

Additional info: Uname: Linux-4.5.0, xorg-server-1.18.2, mesa-11.1.2
Also, just before freezing I was in X11 terminal emulator (with enabled compositing (semitransparent background)) and edited a file in a vim. And there was only Firefox and Thunderbird processes run under in inactive windows.
P.P.S. I'll be happy to provide any additional info on request (although, I still can't figure out exactly conditions to 100% reproducibility)