nouveau: wait to unref the transfer's bo until it's no longer used

nouveau: wait to unref the transfer's bo until it's no longer used

The bo will often come from a slab in which case it doesn't matter. But
for larger allocations this will be in its own bo, and we have to make
sure to wait until it's no longer used in order for it to be freed.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Cc: mesa-stable@lists.freedesktop.org
Tested-by: Marcin Ślusarz <marcin.slusarz@gmail.com>