drm/nouveau: fix booting with plymouth + dumb support

We noticed a plymouth bug on Fedora 18, and I then
noticed this stupid thinko, fixing it fixed the problem
with plymouth.

Cc: stable@vger.kernel.org
Acked-by: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>