[omxplayer] Restore flush/accurate seek flags to fix issue with edl skip

https://github.com/xbmc/xbmc/pull/5084 introduced a regression in edl skipping and make seeking uglier (sometimes decoding from a non-I-frame) when using omxplayer mode.

Fixes this issue: http://forum.xbmc.org/showthread.php?tid=192380&pid=1809229#pid1809229

This PR shouldn't affect dvdplayer behaviour.