Drive-by cleanup, we don't need to validate the port references here as we
already previously went through the effort of refactoring things such that
we're guaranteed to be able to access ->mstb and ->port safely from
drm_dp_check_and_send_link_address(), since the only two places in the
codebase that drop an MST reference in such a way that it would remove it
from the topology are both protected under probe_lock.

Thanks for that, past Lyude!
Signed-off-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Wayne Lin <Wayne.Lin@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220602201757.30431-2-lyude@redhat.com