drm/doc: Fix typos for early_unregister doc

There's no late_unregister. While at it switch to the new canonical
reference style.

Cc: Chris Wilson <[e-mail]>
Acked-by: Chris Wilson <[e-mail]>
Signed-off-by: Daniel Vetter <[e-mail]>
Link: http://patchwork.freedesktop.org/patch/msgid/[e-mail]

include/drm/drm_crtc.h

···
 	 *
 	 * This optional hook should be used to unregister the additional
 	 * userspace interfaces attached to the crtc from
-	 * late_unregister(). It is called from drm_dev_unregister(),
+	 * @late_register. It is called from drm_dev_unregister(),
 	 * early in the driver unload sequence to disable userspace access
 	 * before data structures are torndown.
 	 */

include/drm/drm_encoder.h

···
 	 *
 	 * This optional hook should be used to unregister the additional
 	 * userspace interfaces attached to the encoder from
-	 * late_unregister(). It is called from drm_dev_unregister(),
+	 * @late_register. It is called from drm_dev_unregister(),
 	 * early in the driver unload sequence to disable userspace access
 	 * before data structures are torndown.
 	 */

include/drm/drm_plane.h

···
 	 *
 	 * This optional hook should be used to unregister the additional
 	 * userspace interfaces attached to the plane from
-	 * late_unregister(). It is called from drm_dev_unregister(),
+	 * @late_register. It is called from drm_dev_unregister(),
 	 * early in the driver unload sequence to disable userspace access
 	 * before data structures are torndown.
 	 */