nvc0: do not randomly emit fences.

We track fences in a global list and have a per context "current" fence which we randomly attach things to. If we take such a fence and emit it without also creating a new fence for future tasks we can get out of sync leading to random failures.

Some of our queries could trigger such cases and even though this issues appears to be triggered by the MT rework, I'm convinced that this was only made more visible by those fixes and we had this bug lurking for quite a while.

Closes: #7429 (closed)