drm/nouveau: Don't pick an interlaced mode as the panel native mode.

Rescaling interlaced modes isn't going to work correctly, and even if
it did, come on, interlaced flat panels? are you pulling my leg?

Signed-off-by: Francisco Jerez <currojerez@riseup.net>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>