iommu/arm-smmu: re-use the init_mutex for protecting smmu_domain.smmu

The newly-introduced smmu_domain.init_mutex can be used everywhere that
we're currently using smmu_domain.lock to protect smmu_domain.smmu.
Move to `init_mutex' and delete `lock'.

Change-Id: I92aa237e72cafd0c8fa39fe6279c44bd194c11b4
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>