Yeah, he was knocked out before he hit the ground. Watch his head snap back after crashing into the catwalk in the first place, and then land face first on the floor. If he was still awake at that point, he would have rolled over and moved out of the way before Dooku dropped the thing on him. Or he would have moved when it skidded him a few feet out. but he doesn't even register that something has fallen at any point, so it's pretty easy to see he was knocked out before he ever hit the floor. You don't even have to read the book to figure that.