i965: Only enable __DRI2_ROBUSTNESS if kernel support is available

i965: Only enable __DRI2_ROBUSTNESS if kernel support is available

Rather than always advertising the extension but failing to create a
context with reset notifiction, just don't advertise it.  I don't know
why it didn't occur to me to do it this way in the first place.

NOTE: Kristian requested that I provide a follow-up for master that
dynamically generates the list of DRI extensions instead of selected
between two hardcoded lists.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Suggested-by: Kristian Høgsberg <krh@bitplanet.net>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Kristian Høgsberg <krh@bitplanet.net>
Cc: "10.0" <mesa-stable@lists.freedesktop.org>