lair: robot_knight scene goes too long.

[icculus]

The original arcade version only plays this scene until Dirk is about to enter the tunnel at the back of the room, and omits the piece where he's jumping down the missing stairs after:

https://youtu.be/mjo3OBFbDA4?t=208

The Dragon's Lair Enhancement ROM restores the last piece, and possibly Digital Leisure's builds do, too, but we should tweak this to work like the arcade version by default.