[dvdplayer] don't delete the demuxer all the time, it can be reused
[dvdplayer] use IsCaching() when appropriate
[test] remove CACHESTATE_PVR completely
[dvdplayer] dont call OpenDefaultSteams() on PVR streams, it will
cause all streams to be opened twice in a row (the streams get
discarded once the STREAMCHANGE mux packet comes in a while after)