Comment 7 for bug 1292467

Same here with an i915. Unity was completely unusable after upgrading 12.04 -> 14.04, the internal display which I never used was breaking 3d acceleration. I had the problem with both the Ubuntu default package and the Intel drivers from 01.org. The computer is a low-end PC used as a server, so unplugging the monitor on each reboot is quite inconvenient and using two displays could be useful. In my opinion, this is quite a serious bug.

The "video=LVDS-1:d" workaround fixed the issue, in order to have it permanently, I added the option in /etc/default/grub to the GRUB_CMDLINE_LINUX_DEFAULT variable and ran update-grub.