drm/nouveau: Fix TMDS on some DCB1.5 boards.

The TMDS output of an nv11 was being detected as LVDS, because it uses
DCB type 2 for TMDS instead of type 4.

Reported-by: Bertrand VIEILLE <Vieille.Bertrand@free.fr>
Signed-off-by: Francisco Jerez <currojerez@riseup.net>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>