drm/nouveau: Use drm_dp_get_payload_info() for getting payload/vcpi

Currently, nouveau tries to go through the drm_dp_mst_port structures
itself in order to retrieve the relevant payload and VCPI information
that it needs to report to the GPU. This is wrong: mstc->port could be
destroyed at any point, and additionally the payload could be changed at
any point because it doesn't bother trying to grab the payload lock. So;
remove nv50_msto_payload entirely and use the new
drm_dp_get_payload_info() helper.

Signed-off-by: Lyude Paul <lyude@redhat.com>