glx: fix non-dri build

glXGetDriverConfig parameters do not provide a context to dynamically
check for the presence of the function, so the dispatcher directly calls
glXGetDriverConfig, but in non-dri builds dri_glx.c didn't provide
glXGetDriverConfig.

This change make it just return NULL in that case.

Fixes: 84f764a7 "glxglvnddispatch: Add missing dispatch for GetDriverConfig
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
(cherry picked from commit 47ac11bc)