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

[Mario Kleiner]

Hi

These two patches against radeon and amdgpu ddx are meant to
replace Option "DRI3" with Option "DRI", so the AMD drivers
use the same xorg.conf option for selection between DRI2 and DRI3
as nouveau and the intel driver.

This is tested on the radeon ddx, but only compile tested on
amdgpu due to lack of hw.

thanks,
-mario