iommu/arm-smmu: remove potential NULL dereference on mapping path

When handling mapping requests, we dereference the SMMU domain before
checking that it is NULL. This patch fixes the issue by removing the check
altogether, since we don't actually use the leaf_smmu when creating
mappings.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>