Looks like some of the patches discussed in that bug report work around the problem by disabling PSR-SU for the specific timing controller my display also has. Those patches are in current kernels already. So basically the problem is gone for me, even if I remove the dcdebugmask.

So, I don't really know if the system was fully hanging, or if the display was just unable to update any more, but it was likely exactly the same that happened to other people with Parade TCONs in that bug discussion.