drm/msm: switch msm_kms_init_aspace() to use device_iommu_mapped()

Change msm_kms_init_aspace() to use generic function
device_iommu_mapped() instead of the fwnode-specific interface
dev_iommu_fwspec_get(). While we are at it, stop referencing
platform_bus_type directly and use the bus of the IOMMU device.
Suggested-by: Robin Murphy <robin.murphy@arm.com>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/489703/
Link: https://lore.kernel.org/r/20220616081106.350262-6-dmitry.baryshkov@linaro.orgSigned-off-by: Rob Clark <robdclark@chromium.org>

drivers/gpu/drm/msm/msm_drv.c

@@ -277,21 +277,21 @@ struct msm_gem_address_space *msm_kms_init_aspace(struct drm_device *dev)
 	struct device *mdss_dev = mdp_dev->parent;
 	struct device *iommu_dev;
 
-	domain = iommu_domain_alloc(&platform_bus_type);
-	if (!domain) {
-		drm_info(dev, "no IOMMU, fallback to phys contig buffers for scanout\n");
-		return NULL;
-	}
-
 	/*
 	 * IOMMUs can be a part of MDSS device tree binding, or the
 	 * MDP/DPU device.
 	 */
-	if (dev_iommu_fwspec_get(mdp_dev))
+	if (device_iommu_mapped(mdp_dev))
 		iommu_dev = mdp_dev;
 	else
 		iommu_dev = mdss_dev;
 
+	domain = iommu_domain_alloc(iommu_dev->bus);
+	if (!domain) {
+		drm_info(dev, "no IOMMU, fallback to phys contig buffers for scanout\n");
+		return NULL;
+	}
+
 	mmu = msm_iommu_new(iommu_dev, domain);
 	if (IS_ERR(mmu)) {
 		iommu_domain_free(domain);