iommu: arm-smmu: Work around WZR issue in Qualcomm SMMU

This is a commit based on the diff posted on LKML in the discussion
about Qualcomm SMMU WZR issue, by Robin Murphy. The diff included a
change to arm_smmu_write_context_bank(), which turned out to be
unnecessary.

https://lore.kernel.org/lkml/8eb4f446-6152-ffb6-9529-77fb0bcc307f@arm.com/



Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>