iommu: iommu-debug: check valid pfn before performing ATOS ops

ATOS operation can be fatal when performed on invalid pfn. Validate mapped
entry pfn for validity before performing ATOS operation.

Change-Id: Ie1b118d768c396f91618ee1eb101754cbee5318e
Signed-off-by: Prakash Gupta <guptap@codeaurora.org>
[isaacm@codeaurora: Move PFN validity check prior to IOVA assignment]
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>