https://bugs.freedesktop.org/show_bug.cgi?id=48422
--- Comment #1 from Jonathan Nieder jrnieder@gmail.com 2012-04-07 12:16:24 PDT --- Context: http://bugs.debian.org/667799 which mentions that the system is still usable using ssh after the failed modeset.
Looks like the log with drm.debug=0x6 overflowed the kernel buffer so it's cut off at the beginning. Does /var/log/dmesg have more?