[rbp/omxplayer] Simplify clock handling

In OMXClock we have an almost complete copy of DVDClock, so just use DVDClock directly.

The passing through of CDVDClock functions from OMXClock is hopefully temporary,
as I think clock manipulation is better handled from OMXPlayer than audio/video players.
Merge pull request #2833 from popcornmix/simplify_clock

[rbp/omxplayer] Simplify clock handling