I see the same happening here with a HD4200 (RS880) GPU.

I tried with:

- kernel 3.18rc3
- latest radeon ucode firmware files from the 23rd of August
- updated mesa, xserver and libs from the oibaf ubuntu ppa

I observed the same behavior: video starts well for a second, then the image
freeze (but mouse cursor is still alive) and I can't do anything, not even
switching to another vt. After a while the screen becomes black.

In the meantime I can hear the sound of the video correctly, and indeed if I
log in with ssh from another pc everything is working well; still xserver
doesn't work until a reboot is performed.

I attach a piece of the dmesg log taken from the ssh session right after the
video was played where the GPU hang is reported. Xorg doesn't log any error.