iommu/io-pgtable-arm: Clean up faulty sanity check

Checking for a nonzero dma_pfn_offset was a quick shortcut to validate
whether the DMA == phys assumption could hold at all. Checking for a
non-NULL dma_range_map is not quite equivalent, since a map may be
present to describe a limited DMA window even without an offset, and
thus this check can now yield false positives.

However, it only ever served to short-circuit going all the way through
to __arm_lpae_alloc_pages(), failing the canonical test there, and
having a bit more to clean up. As such, we can simply remove it without
loss of correctness.

Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>