[Mesa-dev] Tidy up dri config options

[Timothy Arceri]

This series removes some unused options, replaces some legacy
debug options with env vars instead and moves a bunch of old
radeon dri driver options directly into the drivers.

I haven't bothered copying all the radeon dri translations
as I didn't want to waste my time as it seems unlikely anyone
will even use those config options these days (I was tempted to
just remove them altogether).

The motivation behind the series is to prep dri conf to be more
modular and eventually use it with the radv driver.