freedreno: multithread fixes

So, turns out duo apk is a pretty good gl multithread fuzzer. Annoyingly effect/filters seem to have stopped working today before I had a chance to confirm that the last patch closed the last race (which seems like something server-side). Playing around with filters for a few minutes seemed to be a reliable way to trigger badness.. but I left duo running for ~10min with no issue so I think the last patch should solve the last remaining issue.