As of kernel 4.1 and mesa 10.6 the system does not crash/hang anymore but I get
the following messages:

radeon: The kernel rejected CS, see dmesg for more information.

and dmesg gives me the following information:

[  422.606081] [drm:radeon_cs_ioctl [radeon]] *ERROR* Invalid command stream !
[  422.658136] [drm:radeon_uvd_cs_parse [radeon]] *ERROR* Invalid UVD handle
0x105c0003!
[  422.658167] [drm:radeon_cs_ioctl [radeon]] *ERROR* Invalid command stream !
[  436.853008] [drm:radeon_uvd_cs_parse [radeon]] *ERROR* Failed mapping the
UVD message (-22)!

This happens on both VLC with VDPAU and VA-API and also on MPV. The video
sometimes plays and sometimes does not (just black).