drm/nouveau: handle same-fb page flips

It's questionable use case, but weston/wayland already relies on this
behaviour, and other drivers don't care about it, so it's a matter of
compatibility.  Without it, process invoking such page flip hangs in
unkillable state, trying to reserve the same buffer twice.

Signed-off-by: Marcin Slusarz <marcin.slusarz@gmail.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>