iommu: tegra/gart: Fix with mainline IOMMU API change

The mainline IOMMU API v3.3-rc1 has been changed so that bytes are
used in mapping size instead of page order and page-by-page iteration
is taken care of by iommu core.

The rest are mostly for maintainability:
$ git co iommu/next drivers/iommu/tegra-gart.c

Change-Id: Id88ae8d23d11f4e003d11ec1e4223a72215ad142
Signed-off-by: Hiroshi DOYU <hdoyu@nvidia.com>
Reviewed-on: http://git-master/r/78138
Reviewed-by: Automatic_Commit_Validation_User
Reviewed-by: Krishna Reddy <vdumpa@nvidia.com>