drm/msm: add IOMMU_SUPPORT dependency

The iommu pgtable support is only available when IOMMU support
is built into the kernel:

WARNING: unmet direct dependencies detected for IOMMU_IO_PGTABLE
  Depends on [n]: IOMMU_SUPPORT [=n]
  Selected by [y]:
  - DRM_MSM [=y] && HAS_IOMEM [=y] && DRM [=y] && (ARCH_QCOM [=y] || SOC_IMX5 || ARM && COMPILE_TEST [=y]) && OF [=y] && COMMON_CLK [=y] && MMU [=y] && (QCOM_OCMEM [=y] || QCOM_OCMEM [=y]=n)

Fix the dependency accordingly. There is no need for depending on
CONFIG_MMU any more, as that is implied by the iommu support.

Fixes: b145c6e6 ("drm/msm: Add support to create a local pagetable")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Rob Clark <robdclark@chromium.org>