drm/amd/display: set NULL value during removal for remoteSink

In MST case during removal of remote sink its descriptor pointer wasn't freed corectly.

Signed-off-by: Leon Elazar <leon.elazar@amd.com>
Acked-by: Harry Wentland <Harry.Wentland@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>