nouveau: avoid emitting new fences unnecessarily

Right now we emit on every kick, but this is only necessary if something
will ever be able to observe that the fence completed. If there are no
refs, leave the fence alone and emit it another day.

This also happens to work around an issue for the kick handler -- a kick
can be a result of e.g. nouveau_bo_wait or explicit kick, or it can be
due to lack of space in the pushbuf. We want the emit to happen in the
current batch, so we want there to always be enough space. However an
explicit kick could take the reserved space for the implicitly-triggered
kick's fence emission if it happened right after. With the new mechanism,
hopefully there's no way to cause two fences to be emitted into the same
reserved space.

Signed-off-by: Ilia Mirkin <[email protected]>
Reviewed-by: Samuel Pitoiset <[email protected]>
Fixes: 47d11990b (nouveau: make sure there's always room to emit a fence)
Cc: [email protected]

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