ASoC: msm: qdsp6v2: Remove obsolete IOMMU domain attribute

The DOMAIN_ATTR_COHERENT_HTW_DISABLE IOMMU domain attribute is being
removed.  SMMU coherency will be configured through the SMMU device tree
nodes moving forward.  Remove the obsolete option.

Change-Id: I6ae3cebd4f3a1f3abc8d84ab407d2e256d55309a
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>