drm/nouveau/kms: handle mDP connectors

In some cases we have the handle those explicitly as the fallback
connector type detection fails and marks those as eDP connectors.

Attempting to use such a connector with mutter leads to a crash of mutter
as it ends up with two eDP displays.

Information is taken from the official DCB documentation.

Cc: stable@vger.kernel.org
Cc: dri-devel@lists.freedesktop.org
Cc: Ben Skeggs <bskeggs@redhat.com>
Reported-by: Mark Pearson <markpearson@lenovo.com>
Tested-by: Mark Pearson <markpearson@lenovo.com>
Signed-off-by: Karol Herbst <kherbst@redhat.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>