egl/dri2: Dispatch eglCreateWindowSurface by display, not driver

Add dri2_egl_display_vtbl::create_window_surface, set it for each
platform, and let egl_dri2 dispatch eglCreateWindowSurface to that.

This prepares for the EGL platform extensions.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Signed-off-by: Chad Versace <chad.versace@linux.intel.com>
diff --git a/src/egl/drivers/dri2/egl_dri2.h b/src/egl/drivers/dri2/egl_dri2.h
index 519aab0..22c8cf5 100644
--- a/src/egl/drivers/dri2/egl_dri2.h
+++ b/src/egl/drivers/dri2/egl_dri2.h
@@ -87,6 +87,11 @@
 struct dri2_egl_display_vtbl {
    int (*authenticate)(_EGLDisplay *disp, uint32_t id);
 
+   _EGLSurface* (*create_window_surface)(_EGLDriver *drv, _EGLDisplay *dpy,
+                                         _EGLConfig *config,
+                                         EGLNativeWindowType window,
+                                         const EGLint *attrib_list);
+
    EGLBoolean (*swap_interval)(_EGLDriver *drv, _EGLDisplay *dpy,
                                _EGLSurface *surf, EGLint interval);