dvd/omxplayer: add internal version of SetSubtitleVisible

This avoids overriding result of OpenDefaultStreams forced/relevant
calculation due to SetSubtitleVisible being delayed by message
queue.

Original-patch-by: ace20022 <ace20022@xbmc.org>
dvd/omxplayer Subs couldn't be turned on if no relevant subs existed

Original patch by: ace20022
dvdplayer: 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.