zink: improve implicit sync on ANV
ANV uses a different mechanism for implicit sync, and since zink has no WSI, different handling is needed in order to ensure that the frontbuffer isn't drawn over before it finishes presenting.
At present, this is going to (counterintuitively) be a very small perf hit on ANV due to how many stupid flushes are happening during each draw call, but that's a separate issue which will be tackled in another MR.