Any response to an attack must be proportional to the attack. If the girl was not handcuffed, overwhelming force might be justified. But the girl was handcuffed. Her threat to the cop was minimal, and so required a measured response from the cop. And that's not what happened here.
Oh, and one more thing. It is immaterial whether or not the girl was mouthing off.
Cardinal rule of brutality is if the girl was handcuffed. She was... thus, his reaction is beyond the scope of what is reasonable. I worked Security at a hospital during college and yes.. we had violent M.O.’s, drunks, druggies, etc. The girl could have been secured to a bed easily or put into 4 point restraints easily. I dealt with situations like that on a daily basis. The cop knew that slamming her head first.. and with all his force into the ground would result in her getting injured. He acted not based on the safety of himself or hospital staff but his OWN anger. He used enough force to knock out two young front teeth.. which means that she also suffered a head injury. If I was in the jury... she would get a lot of $. If I was in the police department, he would turn his papers in. No department needs the type of officer that cannot control their own actions and reactions to something as simple as a missed kick from a drunk.