drm/dp-mst: Drop connection_mutex check

Private atomic objects have grown their own locking with

commit b962a120
Author: Rob Clark <robdclark@gmail.com>
Date:   Mon Oct 22 14:31:22 2018 +0200

    drm/atomic: integrate modeset lock with private objects

which means we're no longer relying on connection_mutex for mst state
locking needs.

Cc: Lyude Paul <lyude@redhat.com>
Cc: Sean Paul <sean@poorly.run>
Reviewed-by: NLyude Paul <lyude@redhat.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191009224113.5432-1-daniel.vetter@ffwll.ch

drivers/gpu/drm/drm_dp_mst_topology.c

@@ -4186,7 +4186,6 @@ struct drm_dp_mst_topology_state *drm_atomic_get_mst_topology_state(struct drm_a
 {
 	struct drm_device *dev = mgr->dev;
 
-	WARN_ON(!drm_modeset_is_locked(&dev->mode_config.connection_mutex));
 	return to_dp_mst_topology_state(drm_atomic_get_private_obj_state(state, &mgr->base));
 }
 EXPORT_SYMBOL(drm_atomic_get_mst_topology_state);