drm/nouveau: Keep malloc references to MST ports

Now that we finally have a sane way to keep port allocations around, use
it to fix the potential unchecked ->port accesses that nouveau makes by
making sure we keep the mst port allocated for as long as it's
drm_connector is accessible.

Additionally, now that we've guaranteed that mstc->port is allocated for
as long as we keep mstc around we can remove the connector registration
checks for codepaths which release payloads, allowing us to release
payloads on active topologies properly. These registration checks were
only required before in order to avoid situations where mstc->port could
technically be pointing at freed memory.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Ben Skeggs <bskeggs@redhat.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: David Airlie <airlied@redhat.com>
Cc: Jerry Zuo <Jerry.Zuo@amd.com>
Cc: Harry Wentland <harry.wentland@amd.com>
Cc: Juston Li <juston.li@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190111005343.17443-15-lyude@redhat.com