gbm_dri: Fix DRIimage lookup callback

gbm_dri: Fix DRIimage lookup callback

That is by making the dri extension variables static in gbm_dri.c.

The image_lookup_extension is provided by egl_dri2 when using x11 or wayland
platforms, when using the drm platform, gbm_dri has a wrapper for it.
Both use the same variables name image_lookup_extension.

Since -fvisibility=hidden was (probably by mistake) removed when converting to
automake, the "image_lookup_extension" symbol from egl_dri2.c became exported
in libEGL.so, so "image_lookup_extension" from gbm_dri.c was ignored.
This resulted in calling incorrect callbacks.

We cant make the image_lookup_extension static in egl_dri2.c right now,
since its used across multiple files.

Bugzilla: https://bugs.freedesktop.org/attachment.cgi?id=58099
Reviewed-by: Kristian Høgsberg <krh@bitplanet.net>