Fix missing identity map check in the material generator

The check was missed in e2c98adf and is needed to correctly select
the right variable name.

Change-Id: Ib0cbb404471b1a27c329910f2b18431b3bb1db94
Reviewed-by: Inho Lee <inho.lee@qt.io>
Reviewed-by: Christian Strømme <christian.stromme@qt.io>