Fill the holes with anchoring compound. Insert cut pieces of threaded rod or similar, and let harden. Then attach the TV mount with nuts.
ah thank you so much for the quick and good responses! I'm thinking we'll go with chemical anchors and threaded rods, it feels safest since the TV weighs quite a lot. And good tip with the wall plugs as well, very useful I think since we're going to put up a small shelf and such 
Again, thanks!
Again, thanks!