iommu/io-pgtable-arm: Simplify start level lookup

Beyond a couple of allocation-time calculations, data->levels is only
ever used to derive the start level. Storing the start level directly
leads to a small reduction in object code, which should help eke out a
little more efficiency, and slightly more readable source to boot.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>