nouveau: avoid double-emitting fence

nouveau: avoid double-emitting fence

The act of ensuring that there is space can cause a flush to happen,
which will emit the current screen fence. If that is the fence we're
trying to wait on, then it will have been emitted as a result of doing
the PUSH_SPACE. Don't attempt to emit it a second time.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Fixes: 8053c9208f (nouveau: avoid emitting new fences unnecessarily)
Cc: mesa-stable@lists.freedesktop.org