drm/amd/display: Fix NULL pointer on MST chained mode

Prevent NULL pointer on new_stream being added to ctx
when added MST connectors cannot be found in existing crtc_state
in the chained mode

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