iris: Convert fences to using lightweight seqno

iris: Convert fences to using lightweight seqno

By using the breadcrumbs we inject into the batch, we can build a
lightweight fence - that can be evaluated in userspace without having to
check in the kernel. In order to pass the fences between processes, and
to wait efficiently, we continue to track the syncobj for each batch and
use that as a terminator for the fence, and for passing coarse
scheduling decisions to the kernel on execbuf.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/3802>