tu: Don't actually track seqno's for events

tu: Don't actually track seqno's for events

We just dropped the last user which actually cared about the seqno.
This never worked anyway, since the seqno was never reset between
multiple executions of the same command buffer. Turn the part of the
control buffer which used to track the seqno into a dummy dword, and
figure out automatically whether we need to include it. We will
implement seqnos again eventually, with timline semaphores, but that
will likely be totally different.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4964>