dri: Pass in the dlsym()ed driver extension to screen creation.

dri: Pass in the dlsym()ed driver extension to screen creation.

This will allow a megadrivers build to reference the actual driver being
loaded from the shared dri_util screen creation code.

v2: Fix indentation, fallback case in EGL (review by Emil).

Reviewed-by: Matt Turner <mattst88@gmail.com> (v1)
Reviewed-by: Chad Versace <chad.versace@linux.intel.com> (v1)
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>