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

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

[ Upstream commit 655c0ed19772d92c9665ed08bdc5202acc096dda ]

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: 4562236b3bc0 ("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>
Signed-off-by: Sasha Levin <sashal@kernel.org>