Since 3.17.3, my system (Dell precision M 6700 notebook with AMD Cap Verde
graphics) locks up hard as soon as I connect a beamer via HDMI
(both when connected to the HDMI output and when connected to the DP output
via a DP/HDMI adapter) while X is running or before starting X
(console mode only is fine). 3.17.2 worked.

With good luck, the system is still alive, but keyboard, display and mouse
are locked up, because X hangs. The system shuts down on soft power off,
but with 10 seconds delay and the filesystems still mounted: 
X seems to be unkillable.

With bad luck, the fan goes to 100 % and the kernel seems to be dead:
Zero disk activity, hard power off is the only way out.

The faulty patch is "drm/radeon: fix speaker allocation setup":
Applying or reverting this patch only makes the difference.