iris: Place a seqno at the end of every batch

iris: Place a seqno at the end of every batch

We can use seqno as a basic for fast userspace fences: where we can
check a value directly to test for fence completion without having to
query using the kernel. To do so we need to write a breadcrumb from the
batch and track those writes as the basis for our lightweight fences.

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