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: Lyude Paul <lyude@redhat.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191009224113.5432-1-daniel.vetter@ffwll.ch