rtsp/rdt: Assign the RTSPStream index to AVStream->id

This is used for mapping AVStreams back to their corresponding
RTSPStream. Since d9c0510e, the RTSPStream pointer isn't stored in
AVStream->priv_data any longer, breaking this mapping from AVStreams
to RTSPStreams.

Also, we don't need to clear the priv_data in rdt cleanup any longer,
since it isn't set to duplicate pointers.

Signed-off-by: Ronald S. Bultje <rsbultje@gmail.com>
(cherry picked from commit b2dd842d)