drm/nouveau: Try to fetch an EDID from OF if DDC fails.

More Apple brain damage, it fixes the modesetting failure on an eMac
G4 (fdo bug 29810).

Reported-by: Zoltan Varnagy <doi@freemail.hu>
Signed-off-by: Francisco Jerez <currojerez@riseup.net>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>