Does the Dawn Prayer become invalidated by masturbating after returning from the mosque due to watching a pornographic film, and must it be repeated?

Your prayer is valid as long as you performed it while in a state of purity. What is incumbent upon you is to repent to Allah and to cease watching the vile movies that increase sins, distance you from Allah, and cause a stain on the heart. So, fear Allah and be shy before Him, and remember that He sees you, and do not make Him the least significant of those who watch you.