nouveau: annotate the private symbols

They are less and easier to track than the public ones. The macro
drm_public will be going away by the end of the series.

Cc: Maarten Lankhorst <maarten.lankhorst@canonical.com>
Cc: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>