drm/nouveau/gem: use bo.offset rather than mm_node.start

Won't necessarily be a drm_mm_node in the future, and I can't think of any
good reason to not use the offset from the bo struct.  There may have been
some reason once apon a time, but, separate commit just in case.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>