iommu: msm: Fix the tough checks in get_attr

Dynamic domains will never be _properly_ attached
to any CB. That means, we cannot get a hold on CB
given a dynamic domain. And that is fine. So, don't
restrict clients to get TTBR0 and CONTEXTIDR from
dynamic domain.

Change-Id: I9a28819d6ab1281e4dd1a5e870a773bbab37ec40
Signed-off-by: Chintan Pandya <cpandya@codeaurora.org>