lavapipe: skip "pipeline barriers" if they're first or last in a cmdbuf

lavapipe: skip "pipeline barriers" if they're first or last in a cmdbuf

barriers just trigger a full flush, which is unnecessary if it's:
* the first cmd in a cmdbuf, as the previous cmdbuf will have already
  flushed and so this is flushing nothing
* the last cmd in a cmdbuf, as there will be a flush immediately after
  returning from this function

Reviewed-by: Dave Airlie <airlied@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10762>