drivers/dri: cleanup dri extension instantiation

drivers/dri: cleanup dri extension instantiation

Uniformly use the typecasted extension name, constify extension instances
and use designated initialisers. Set the implemented version of the
extension, over the one defined in dri_infertace.h. Patch covers the
following extensions:

__DRItexBufferExtension
__DRIimageExtension
__DRIrobustnessExtension
__DRI2rendererQueryExtension
__DRIdri2LoaderExtension

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>