dri2: Require working buffer invalidation
The drivers now require a loader with a working useInvalidate hook, which corresponds to Mesa 8.0. The server side has been in place since DRI2 minor version 3 in xserver 1.6.0, various drivers took their time getting there though. In particular, the intel ddx's uxa backend still does not support this, which is not the default (and has not been since 2013) but does still exist for some reason.
Tagging this with i915 since that (gen2/3 specifically) still defaults to DRI2, at least until !9734 (merged) happens.