drm/msm: stop supporting no-IOMMU configuration

With the switch to GPUVM the msm driver no longer supports the no-IOMMU
configurations (even without the actual GPU). Return an error in case we
face the lack of the IOMMU.

Fixes: 111fdd2198e6 ("drm/msm: drm_gpuvm conversion")
Signed-off-by: Dmitry Baryshkov <[e-mail]>
Patchwork: https://patchwork.freedesktop.org/patch/672559/
Signed-off-by: Rob Clark <[e-mail]>

drivers/gpu/drm/msm/msm_kms.c

···
 		iommu_dev = mdp_dev;
 	else
 		iommu_dev = mdss_dev;
-
 	mmu = msm_iommu_disp_new(iommu_dev, 0);
 	if (IS_ERR(mmu))
 		return ERR_CAST(mmu);
 
 	if (!mmu) {
-		drm_info(dev, "no IOMMU, fallback to phys contig buffers for scanout\n");
-		return NULL;
+		drm_info(dev, "no IOMMU, bailing out\n");
+		return ERR_PTR(-ENODEV);
 	}
 
 	vm = msm_gem_vm_create(dev, mmu, "mdp_kms",