egl: annotate dri2_egl_display_vtbl as const data

With the final place that modifies the vtbl removed as of last commit we
can annotate the symbols accordingly.

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>
diff --git a/src/egl/drivers/dri2/platform_drm.c b/src/egl/drivers/dri2/platform_drm.c
index 0e16d52..50627a7 100644
--- a/src/egl/drivers/dri2/platform_drm.c
+++ b/src/egl/drivers/dri2/platform_drm.c
@@ -646,7 +646,7 @@
    return (count != 0);
 }
 
-static struct dri2_egl_display_vtbl dri2_drm_display_vtbl = {
+static const struct dri2_egl_display_vtbl dri2_drm_display_vtbl = {
    .authenticate = dri2_drm_authenticate,
    .create_window_surface = dri2_drm_create_window_surface,
    .create_pixmap_surface = dri2_drm_create_pixmap_surface,