Created attachment 108839 [details]
dmesg output with the relevant call trace

With kernel version 3.17.2, when I connect my HDMI monitor, it shows up
correctly in xrandr. But when I try to enable it, Xorg freezes and becomes a
zombie process, and the HDMI monitor stays blank. dmesg hints radeon kernel
code is at fault (attached).

Luckily, the system does not completely freeze. I can still connect to the
system through ssh. So I can provide more information if needed.

I remember having similar issues with 3.16 when I tried it. I'm currently using
3.15.5 which does not suffer from this bug.