msm: kgsl: 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: I4bbbfb90fe172e048cc9504c4e6be5f36501b485
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>