Push __driDriverExtensions out of dri_util.c and into the drivers

Push __driDriverExtensions out of dri_util.c and into the drivers

This lets the individual drivers select which extensions to advertise.
Specifically, most drivers (tdfx, sis, savage etc) don't support DRI2
but the shared extension list in dri_util.c does list the DRI2 extension.

Pushing the list into the drivers, lets us avoid listing the DRI2
extension for drivers that don't support it.