drm/nouveau: only runtime suspend by default in optimus configuration

The intent was to only enable it by default for optimus, e.g. see the
runtime_idle callback. The suspend callback may be called directly, e.g.
as a result of nouveau_crtc_set_config.

Reported-by: Stefan Lippers-Hollmann <s.l-h@gmx.de>
Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Tested-by: Stefan Lippers-Hollmann <s.l-h@gmx.de>
Cc: stable@vger.kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>