Answer: When Jack leans into the door to say 'Here's Johnny', there is only one panel missing. When we see Jack and the door after we realize Dick is arriving, there are two panels knocked out.
Either Jack was very productive while the camera cut away to show Dick, or this is a goof. Or, I suppose, it all has some deeper meaning that I just don't buy into. Apparently, the first doors they had built for these scene were like stunt doors, built flimsy to facilitate work of the 'movie star' who had to break through them. What they didn't know was that Jack Nicholson used to be a volunteer firefighter and know how to use an axe. He went through the first doors with way to much strength and they shattered. The carpenters ended up having to build a whole new batch of doors (enough for Kubrick's obscene number of takes) that would be stronger and allow Jack to really lay into them.