Use GE state accessors more often

This doesn't fix every single register, but it cleans up most of the ones we do use.  @Orphis, hopefully `isClutIndexSimple()` horrifies you less, heh.

Should be typesafe.  Tried to keep the softgpu changes as separate commits to hopefully reduce conflicts/merge hassle.  Sorry if there's a bit, @neobrain, I wanted to keep things using enums where possible.

-[Unknown]