iris: Fix memory leaks caused by accumulating way too many syncobj references

Kenneth Graunke requested to merge kwg/mesa:fence-leak-fix into master

Applications which didn't use compute but did use glWaitSync were accumulating way too many refcounts on render batch syncobjs, causing the kernel to hold on to those syncobj's and associated state for way too long. This cleans up that memory leak. See the final patch's commit message for more details.

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