msm: kgsl: Remove code for CONFIG_QCOM_IOMMU

In older kernel versions, CONFIG_QCOM_IOMMU was used to identify
the QCOM version of the IOMMU that some versions of the graphics
driver used.  In 4.19 the same config option is used to idenitfy
a different version of a QCOM iommu driver but not the one that
KGSL expects. Remove this unused code.

Change-Id: Ic0dedbadb33cce4e8aa0b7ccac7ae2e67bd6bd92
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>