i965/gen4: Move URB fence recalculate to emit() time.

This is used by the unit state, which is at emit() time.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Acked-by: Paul Berry <stereotype441@gmail.com>

src/mesa/drivers/dri/i965/brw_urb.c

@@ -216,7 +216,7 @@ const struct brw_tracked_state brw_recalculate_urb_fence = {
       .cache = (CACHE_NEW_VS_PROG |
 		CACHE_NEW_SF_PROG)
    },
-   .prepare = recalculate_urb_fence
+   .emit = recalculate_urb_fence
 };