[Nouveau] concern about bo "pinned'ness" after gem pushbuf is done

[Maarten Maathuis]

As far as i see, we reserve the bo in the gem pushbuf ioctl, and
unreserve after the pushbuf and the fence have been emitted. This
assumption holds if bo moves happen on the same channel or if a cpu
copy happens and we're waiting on fences. But for nv50 it is very
common for the bo move to happen on the kernel fifo, which does not
guarantee anything. Is there any safety i missed?

Maarten.