drm/amd/dc: Fix a missing check bug in dm_dp_mst_detect()

In dm_dp_mst_detect(), We should check whether or not @connector
has been unregistered from userspace. If the connector is unregistered,
we should return disconnected status.

Fixes: 4562236b ("drm/amd/dc: Add dc display driver (v2)")
Signed-off-by: Yingjie Wang <wangyingjie55@126.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>