nouveau: fix fence waiting logic in screen destroy

nouveau: fix fence waiting logic in screen destroy

nouveau_fence_wait has the expectation that an external entity is
holding onto the fence being waited on, not that it is merely held onto
by the current pointer. Fixes a use-after-free in nouveau_fence_wait
when used on the screen's current fence.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=75279
Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Christoph Bumiller <e0425955@student.tuwien.ac.at>
Cc: "9.2 10.0 10.1" <mesa-stable@lists.freedesktop.org>