bridge_softmix: Fix sporadic incorrect video stream mapping.

When an externally initiated renegotiation occurred it was
possible for video streams to be incorrectly remapped,
resulting in no video flowing to some receivers.

This change ensures that only the video source sets up
mappings and also that removed streams do not have mappings
set up.

Change-Id: Iab05f2254df3606670774844bb0935f833d3a9b0