drm/msm: Fix link error with !MSM_IOMMU

The DRM driver for MSM depends on symbols from the MSM
IOMMU driver. Add this dependency to the Kconfig file.

Fixes this comile error:

  Kernel: arch/arm/boot/zImage is ready
  ERROR: "msm_iommu_get_ctx" [drivers/gpu/drm/msm/msm.ko]
  undefined!
  make[2]: *** [__modpost] Error 1

Cc: Rob Clark <robdclark@gmail.com>
Signed-off-by: Joerg Roedel <joro@8bytes.org>