iommu/arm-smmu: fix capability checking prior to device attach

If somebody attempts to check the capability of an IOMMU domain prior to
device attach, then we'll try to dereference a NULL SMMU pointer through
the SMMU domain (since we can't determine the actual SMMU instance until
we have a device attached).

This patch fixes the capability check so that non-global features are
reported as being absent when no device is attached to the domain.

Change-Id: I562cddecc000cec1059d232decc89ee0d6746850
Signed-off-by: Will Deacon <will.deacon@arm.com>
Git-commit: d3bca16635ae7443139c4408def7c1a50755083f
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/will/linux.git


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