drm/i915: Don't send hotplug in intel_dp_check_mst_status()

This hotplug also isn't needed: drm_dp_mst_topology_mgr_set_mst()
already sends a hotplug on its own from drm_dp_destroy_connector_work()
after destroying connectors in the MST topology.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Cc: Imre Deak <imre.deak@intel.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190129191001.442-4-lyude@redhat.com