Have you done anything to your pbuilder setup (pbuilderrc, gbp.conf) to make it install/use beignet?The only thing that might give a hint is BINDMOUNTS="/dev/shm" I did not take any special means for beignet and I think I should not since autobuilders will not have this as well - or am I wrong here?
To use beignet inside a chroot, you'd need it installed inside (e.g. via EXTRAPACKAGES) and BINDMOUNTS=/dev/dri, so whatever is messing up your screen, it's unlikely to be the build using beignet.
Do you have any background tasks running that would be using beignet?(You're correct that only pocl is likely to work on an autobuilder, and is hence the preferred option for build-time OpenCL test suites: server processors usually omit the integrated GPU in favour of more CPU cores.)