iommu/arm-smmu-v3: limit reporting of MSI allocation failures

Currently, the arm-smmu-v3 driver expects to allocate MSIs for all SMMUs
with FEAT_MSI set. This results in unwarranted "failed to allocate MSIs"
warnings being printed on systems where FW was either deliberately
configured to force the use of SMMU wired interrupts -or- is altogether
incapable of describing SMMU MSI topology (ACPI IORT prior to rev.C).

Remedy this by checking msi_domain before attempting to allocate SMMU
MSIs.

Acked-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Nate Watterson <nwatters@codeaurora.org>
Signed-off-by: Sinan Kaya <okaya@codeaurora.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>