drm/nouveau: Support render nodes

Enable support for drm render nodes for nouveau by flagging the ioctls that
are safe and just needed for rendering.

Cc: Ben Skeggs <bskeggs@redhat.com>
Cc: Maarten Lankhorst <maarten.lankhorst@canonical.com>
Signed-off-by: Martin Peres <martin.peres@labri.fr>
Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>