gallium: annotate drm_driver_descriptor instance as const data

Already treated and handled as such.

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>

src/gallium/include/state_tracker/drm_driver.h

@@ -108,13 +108,13 @@ struct drm_driver_descriptor
    const struct drm_conf_ret *(*configuration) (enum drm_conf conf);
 };
 
-extern struct drm_driver_descriptor driver_descriptor;
+extern const struct drm_driver_descriptor driver_descriptor;
 
 /**
  * Instantiate a drm_driver_descriptor struct.
  */
 #define DRM_DRIVER_DESCRIPTOR(driver_name_str, func, conf) \
-struct drm_driver_descriptor driver_descriptor = {             \
+const struct drm_driver_descriptor driver_descriptor = {       \
    .driver_name = driver_name_str,                             \
    .create_screen = func,                                      \
    .configuration = (conf),				       \