iommu/omap: Use dev_get_drvdata()

Using dev_get_drvdata directly.

Cc: Joerg Roedel <joro@8bytes.org>
Cc: iommu@lists.linux-foundation.org
Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>