drm/nouveau: Stop reading port->mgr in n50_mstc_get_modes()

mstc->port isn't validated here so it could be null or worse when we
access it. And drivers aren't ever supposed to be looking at it's
contents anyway. Plus, we can already get the MST manager from
&mstc->mstm->mgr.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Cc: stable@vger.kernel.org