st/dri: cleanup dri extension handling

st/dri: cleanup dri extension handling

Explicitly set the version that is implemented, as that may differ
from the one defined in dri_interface.h. Use designated initialisers
and constify whereever possible.

Note: __DRIimageExtension should not be made const as it's modified
at runtime. This patch should have no side effects on compilers that
do not support designated initialisers, as the existing code in
dri/common already uses them.

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Kristian Høgsberg <krh@bitplanet.net>