Quick fix for CGA configurations

In my efforts to clean up Video component initialization, I inadvertently broke video configs that don't specify a model. Those configs rely instead on the Chipset "monitor type" switch settings to determine whether the video card should initialize as an MDA or a CGA. You can imagine such machines as having both cards installed, and the switches determine which card is attached to your primary (only) display.

Going forward, video configs should always specify a model (one of: "mda", "cga", "ega" or "vga"), especially in "dual display" configurations. It's still OK to rely on switch settings -- and you can still toggle the "monitor type" switches, reset the machine, and change the video config on the fly -- but only when using exactly one of the older cards (ie, MDA or CGA).