iris: Add infrastructure to partition batch into sync boundaries.

iris: Add infrastructure to partition batch into sync boundaries.

This introduces some minimalistic infrastructure which will be used in
order to partition the batch into a series of sections, each one with
a unique, monotonically-increasing sequence number.  Section
boundaries will typically lie at points in the batch where the
execution and memory coherency status of some previous commands are
known, e.g. at batch buffer boundaries or PIPE_CONTROL commands.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/3875>