dvdplayer: simplify calculation of playing pts for audio

see title.
This is the first change of an initiative to improve beginning of playback. Currently audio player starts with an error and changes dvdclock. In particular for large errors changing the clock does harm. Sometimes it results in fps dropping below 10. IMO it is wrong that faulty audio ruins video as well. I will mute audio in such cases so audio can sync to reference clock without killing video.