iommu/vt-d: Cleanup after converting to dma-iommu ops

Some cleanups after converting the driver to use dma-iommu ops.
- Remove nobounce option;
- Cleanup and simplify the path in domain mapping.

Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Tested-by: Logan Gunthorpe <logang@deltatee.com>
Link: https://lore.kernel.org/r/20201124082057.2614359-8-baolu.lu@linux.intel.com


Signed-off-by: Will Deacon <will@kernel.org>