drm/nouveau: use NULL for pointer assignment.

Replace the use of 0 in the pointer assignment with NULL to address the
following sparse warning:
drivers/gpu/drm/nouveau/nouveau_hwmon.c:744:29: warning: Using plain integer as NULL pointer

Signed-off-by: Wambui Karuga <wambui.karugax@gmail.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>