sparc64: move dereference after check for NULL

We shouldn't dereference "iommu" until after we have checked that it is
non-NULL.

Fixes: f08978b0 ("sparc64: Enable sun4v dma ops to use IOMMU v2 APIs")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>