iris: Store a seqno for each batch in the fence

iris: Store a seqno for each batch in the fence

In the next patch, we will introduce deferred fences where we will need
to flush a fence later. To do this, we need to know which batch requires
flushing, so keep a 1:1 mapping between seqno[] and the associated
batch.

It's also substantially less confusing to have a 1:1 mapping.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/3802>