msm: mdss: 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: Ie1dbb942949265a06a8b091f390610ef56cc29d2
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>