drm/nouveau: Fix DPMS 1 on G4 Snowball, from snow white to coal black.

Fixes https://bugs.freedesktop.org/show_bug.cgi?id=40275

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Signed-off-by: Stefan de Konink <stefan@konink.de>
Signed-off-by: Marcin Slusarz <marcin.slusarz@gmail.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>