arm64: dma-mapping: Cocci spatch "vma_pages"

Use vma_pages function on vma object instead of explicit computation.
Found by coccinelle spatch "api/vma_pages.cocci"

Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Thomas Meyer <thomas@m3y3r.de>
Signed-off-by: Will Deacon <will.deacon@arm.com>