iris: Drop comment about ISP_DIS

i965 re-emits 3DSTATE_CONSTANT_* on every batch, so there's no point in
restoring the constants from the context.  Iris actually re-pins the
constant buffers properly across the batch, and avoids re-emitting the
constant packets unless it's necessary.  So, we don't want ISP_DIS.

src/gallium/drivers/iris/iris_batch.c

@@ -444,8 +444,6 @@ iris_chain_to_new_batch(struct iris_batch *batch)
 static void
 iris_finish_batch(struct iris_batch *batch)
 {
-   // XXX: ISP DIS
-
    /* Emit MI_BATCH_BUFFER_END to finish our batch. */
    uint32_t *map = batch->map_next;