iris: Fix accidental busy-looping in query waits
When switching from bo_wait to sync-points, I missed that we turned an
if (not landed) bo_wait into a while (not landed) check_syncpt(), which
has a timeout of 0. This meant, rather than sleeping until the batch
is complete, we'd busy-loop, continually asking the kernel "is the batch
done yet???". This is not what we want at all - if we wanted a busy
loop, we'd just loop on !snapshots_landed. We want to sleep.
Add an effectively infinite timeout so that we sleep.