drm/amd/display: Check dc_sink every time in MST hotplug

Extended fix to: "Don't read EDID in atomic_check"

Fix issue of missing dc_sink in .mode_valid in hot plug routine.

Need to check dc_sink everytime in .get_modes hook after checking
edid, since edid is not getting removed in hot unplug but dc_sink
doesn't.

Signed-off-by: Jerry (Fangzhi) Zuo <Jerry.Zuo@amd.com>
Reviewed-by: Roman Li <Roman.Li@amd.com>
Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org