There is the possibility that a fix is available with -current today, and will be part of 5.2-release, expected on or about 1 November 2012.
You could test this by installing or upgrading a -current snapshot. Be sure to back up your system, upgrades are one-way, you will not be able to downgrade.
Code:
revision 1.121
date: 2012/05/21 20:14:18; author: kettenis; state: Exp; lines: +4 -1
Always clear out the inactive list and make sure everything is unbound in
i915_gem_idle(). If we don't do that we end up with stale entries on the
inactive list which will panic the kernel the next time we use the list.
Fixes restarting X on Sandy Bridge.