iommu/arm-smmu: Remove domain lock variable

The `lock' field of struct arm_smmu_domain was replaced by `init_mutex'
in 9725ec12 (iommu/arm-smmu: re-use the init_mutex for protecting
smmu_domain.smmu), but the `lock' field itself was not deleted.  It's
not meant to be used anymore, so delete it.  Some usages of the crufty
lock have also crept up, so fix those as well.

Change-Id: I33c2f83e7b15f0ec2cb08c784a84991a7c57950f
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>