drm/nouveau: Fix fbcon on mixed pre-NV50 + NV50 multicard.

We used single shared fbops struct and patched it at fb init time with
pointers to the right variant. On mixed multicard, this meant that
it was either sending NV50-style commands to all cards, or NV04-style
commands to all cards.

Signed-off-by: Marcin Kościelnicki <koriakin@0x04.net>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>