Switch Option DRI3 to DRI for consistency with other video drivers. (v2)

[Mario Kleiner]

Ok, updates series, taking Michels feedback into account.
Now still honors Option DRI3, but overrides with Option DRI
if both are present, and checks Option DRI for valid levels 2 and 3.

All combos tested on xf86-video-ati.

thanks,
-mario