defconfig: disable CONFIG_MSM_IOMMU

Change-Id: I014eaf72689ba3ecacec890b85255312bdc54722
kgsl_iommu.c: Fix warning: 'get_iommu_unit' defined but not used
[-Wunused-function]
error, forbidden warning: kgsl_iommu.c:31

Change-Id: I7c469042804be0e269e3579ce13a9af914d6dbe1