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>
(cherry picked from commit 1d8cba9b51b7a6e7dbf3f0d3f53b5c232fd0b5b2)