dvd/omxplayer Subs couldn't be turned on if no relevant subs existed

Original patch by: ace20022
dvd/omxplayer: don't override video setting visibility if no subs are found

Note: This should be reverted when we have separated GUI display from
user specified settings.

It temporarily solves the use case of user starting a movie without subs,
stop it, add external subtitles, start it again.