drm/radeon: Fix a missing check bug in radeon_dp_mst_detect()

In radeon_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: 9843ead0 ("drm/radeon: add DisplayPort MST support (v2)")
Signed-off-by: Yingjie Wang <wangyingjie55@126.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>