drm/nouveau: Workaround missing GPIO tables on an Apple iMac G4 NV18.

This should fix the reported TV-out load detection false positives
(fdo bug 29455).

Reported-by: Vlado Plaga <rechner@vlado-do.de>
Signed-off-by: Francisco Jerez <currojerez@riseup.net>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>