ARM: dts: msm: Disable the coherent KGSL IOMMU test device

To prevent the possibility of running out of stream-matching
entries (SMEs) on the KGSL SMMU in cases where all the SMEs
are required, disable the coherent KGSL IOMMU test device.

Change-Id: I1d1cf1a4530680de91c839d22f057b807765111d