drm/amd/display: Fix active dongle hotplug

Clean fake sink flag after detecting link on downstream port.
Fixing display light-up after  "hot-unplug&plug again" downstream
of an active dongle.

Signed-off-by: Roman Li <Roman.Li@amd.com>
Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>