iommu: dma-mapping-fast: 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: I0e3d3a134b686dd5b639827ef4ee80c4d31df613
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>