The only user was radeon/r200, which was using it to have something that
looks a lot like an enum value return a float from the config option.
Just convert that option to a plain float value (for compat with existing
driconfs) with the min and max of its disjoint range as the range.  The
driver's option handling code already correctly deals with other values in
the range.

The disjoint range support was a bunch of extra parsing for this dead
driver, and made turning driconf into static structs difficult.

Reviewed-by: Eric Engestrom <eric@engestrom.ch>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6916>