iommu: io-pgtable-arm: Improve coverage of arm_lpae_range_has_mapping

The arm_lpae_range_has_mapping currently checks if there are any
non-mapped slots in a given iova range, but it's really meant to check
if there is *any* mapping whatsoever in a given iova range.  Fix this.

Change-Id: I90e426ab157cc194328b754ac5021051ac883603
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>