pcm-dvd: Support channel configuration changes

The sample buffering logic does not take into account that the blocksize
could change. Reset the buffer if the channel configuration changes,
since if there are leftover samples, it is most likely a broken or
misconcatenated stream. This could lead to negative numbers for
missing_samples during decoding.

Thanks to Michael Niedermeyer for pointing these out.
pcm-dvd: Minor leftovers

Drop a pointless branch in uninit and use the compact copyright.