Most TV mounts with swing arms have bolts that can be tightened at the pivot points. Remove the plastic covers and tighten slightly until your TV stays in place.
I can't see your setup but I drilled holes in the arms of a couple of mounts and used sliding patio door locks to keep them in place.
Our big set is on a slanted surface and gravity makes it extend. The previous owner ran nylon straps from the face frame to the side of the set and installed smaps ont the strap and TV housing.
I don't like the set but too lazy to change it and install hidden lock.