iris: simplify how we handle workaround_bo

Here's a small series that changes a little bit our workaround_bo case handling. The big gains here are the fact that now we return early from iris_use_pinned_bo() every time the workaround_bo is passed to it (which includes during pipe_control time). This saves us the atomic operations in iris_bo_bump_seqno() and the possibly big iteration at find_validation_entry().