Bug#948288: gnome-shell reproducibly crashes on boot, and the boot process gets stuck

[Md Ayquassar]

I added nomodeset as described in https://askubuntu.com/a/38834 . Then, the boot
process displayed some error concerning UMS and radeon early in the process, and
the screen resolution is different. The boot process still gets stuck, and
switching with Ctrl+Alt+F1 followed by Ctrl+Alt+F2 makes it continue, but the
segfault message disappears from dmesg.