drm/amd/display: assign new stream id in dc_copy_stream

[why]
stream id should uniquely identify an instance of stream.
the copy constructor should be treated as a new
stream instance.

[how]
assign a new stream id in this copy constructor.

Signed-off-by: Wenjing Liu <Wenjing.Liu@amd.com>
Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>