drm/nouveau/kms: disallow dual-link harder if hdmi connection detected

The fallthrough cases (pre-Fermi) would accidentally allow dual-link pixel
clocks even where they shouldn't be.  This leads to a high resolution HDMI
displays, connected via a DVI->HDMI adapter, to fail on the original NV50.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>