[rbp/omxplayer] Reduce the size of preroll

The preroll defines how much earlier the clock starts compares to timestamps of audio/video.
This needs enough time for GPU side worst video/audio decode/schedule/render.
This should never be more than 200ms.

This makes playback start and seeks 600ms more responsive which is noticable when seeking through a video.
Merge pull request #2929 from popcornmix/reduce_preroll

[rbp/omxplayer] Reduce the size of preroll