msm: kgsl: rename iommu_ops to avoid clashing with generic code

A global variable named `iommu_ops' is defined in generic ARM
code (arch/arm/mm/dma-mapping.c) when CONFIG_ARM_DMA_USE_IOMMU=y. Rename
the kgsl variable to avoid a clash.

Change-Id: I18e36e1224b56eff7c5e03a28cc8b517d65c6dbb
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>