I'm also affected by this bug with my r7 260x on Ubuntu 16.04. If I'm able to
switch to a tty my system give my an endless amount of messages with "radeon
000:01:00.0: ring 0 stalled for more than 10004msec". Last time I also got 
"[drm:ci_dpm_enable [radeon]] *ERROR* ci_start_dpm failed
[drm:radeon_pm_resume [radeon]] *ERROR* radeon: dpm resume failed
[drm:radeon_pm_resume [radeon]] *ERROR* radeon: dpm resume failed"