drm/nouveau: remove quirk to fabricate DVI-A output on DCB 1.5 boards

There's a report of this quirk breaking modesetting on at least one board.
After discussion with Francisco Jerez, we've decided to remove it:

<darktama> it's not worth limiting the quirk to just where we know it can
           work?  i'm happy either way really :)
<curro> hmm, don't think so, most if not all DCB15 cards have just one DAC
<curro> and with that quirk there's no way to tell if the load comes from
        the VGA or DVI port

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