iommu/dma: Factor out remapped pages lookup

Since we duplicate the find_vm_area() logic a few times in places where
we only care aboute the pages, factor out a helper to abstract it.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
[hch: don't warn when not finding a region, as we'll rely on that later]
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Joerg Roedel <jroedel@suse.de>