drm/dp_mst: Get rid of old comment in drm_atomic_get_mst_topology_state docs

We don't actually care about connection_mutex here anymore, so let's get
rid of the comment mentioning it in this function's kdocs.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Wayne Lin <Wayne.Lin@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220602201757.30431-4-lyude@redhat.com