iris: Drop comment about ISP_DIS

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.