drm/msm/mdp4: 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 for an MDP4 device.

Fixes: 111fdd2198e6 ("drm/msm: drm_gpuvm conversion")
Signed-off-by: Dmitry Baryshkov <[e-mail]>
Patchwork: https://patchwork.freedesktop.org/patch/672557/
Signed-off-by: Rob Clark <[e-mail]>

drivers/gpu/drm/msm/disp/mdp4/mdp4_kms.c

···
 		ret = PTR_ERR(mmu);
 		goto fail;
 	} else if (!mmu) {
-		DRM_DEV_INFO(dev->dev, "no iommu, fallback to phys "
-				"contig buffers for scanout\n");
-		vm = NULL;
+		DRM_DEV_INFO(dev->dev, "no IOMMU, bailing out\n");
+		ret = -ENODEV;
+		goto fail;
 	} else {
 		vm  = msm_gem_vm_create(dev, mmu, "mdp4",
 					0x1000, 0x100000000 - 0x1000,