glxglvnddispatch: Add missing dispatch for GetDriverConfig

glxglvnddispatch: Add missing dispatch for GetDriverConfig

Together with some fixes to xdriinfo this fixes xdriinfo not working
with glvnd.

Since apps (xdriinfo) expect GetDriverConfig to work without going to
need through the dance to setup a glxcontext (which is a reasonable
expectation IMHO), the dispatch for this ends up significantly different
then any other dispatch function.

This patch gets the job done, but I'm not really happy with how this
patch turned out, suggestions for a better fix are welcome.

Cc: Kyle Brenneman <kbrenneman@nvidia.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Cc: mesa-stable@lists.freedesktop.org