Apparently after several kernel versions later hibernate is still broken on my
machine. :(

uname -a: Linux HyperArchMachine 4.6.3-1-ARCH #1 SMP PREEMPT Fri Jun 24
21:19:13 CEST 2016 x86_64 GNU/Linux
Operating system is Arch/Antergos.

Sympthoms:
-No panic is displayed. The machine freezes completly soon as radeon tries to
hibernate (to swap disk 8GiB). No way to soft reset.
-On resuming first display VTs are showing up weird: resolution is same as
second display (1680x1050) but parts beyond that reso of the FullHD display are
corrupted (pink pixel noise).
-[drm:si_dpm_set_power_state [radeon]] *ERROR* si_set_sw_state failed gets
outputed on suspend/resume.

Disabling DPM on boot allows machine to hibernate/resume correctly but dual
display VT weirdness persists.