Simplify auto device configuration for choosing wsfb, fbdev

I wanted to simplify the logic, and thought this is a good opportunity
to eliminate local diffs.

I don't want to list OSes without wsfb, because I understand that is a
netbsd/openbsd driver, and always have it as a fallback for us.

Additionally, I understand "fbdev" is linux-specific, so have the logic
match this intent.