(In reply to comment #1)
> Created attachment 68742 [details]
> Debian dmesg output, which does have this problem
[...]
> [    5.773063] ni_cp: Failed to load firmware "radeon/BARTS_pfp.bin"
> [    5.773113] [drm:evergreen_startup] *ERROR* Failed to load firmware!

Is the firmware-linux-nonfree package installed on Debian Wheezy?