nouveau: safen up nouveau_device list usage against concurrent access

I cannot make nouveau_bo_wrap thread-safe (by design), but it seems to be used to convert
drm fb's to nouveau_bo's and to get a notify handle from fifo->notify in nv30_screen.c

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@ubuntu.com>