drm/amd/display: fix usb-c dongle regression

A previous change moved link_disconnect_sink after dpcd read,
this causes the sink count to be overwritten which in turn causes
another detection to be triggered in the short pulse handler, which
brings down the display.

Signed-off-by: Eric Yang <Eric.Yang2@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>