demux: reduce wakeups if no cache is active

The purpose of the unconditional pthread_cond_signal() when reading
cached DEMUXER_CTRLs and STREAM_CTRLs was apparently to update the
stream cache state. Otherwise, the cached fields would never be updated
when the stream is e.g. paused.

The same could be said about other CTRLs, but these aren't as important,
since they are normally updated while reading packet data.

In order to reduce wakeups, make this logic explicit.

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