drm/amd/display: Reboot while unplug hdcp enabled dp from mst hub

[Why]
event_property_update does not check NULL pointer

[How]
check aconnector->base.state equals NULL

Reviewed-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Acked-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: hersen wu <hersenxs.wu@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>