drm/tegra: dc: Detach IOMMU group from domain only once

Detaching from an IOMMU group multiple times can lead to a crash. This
could potentially be fixed in the IOMMU driver, but it's easy to avoid
the subsequent detach operations in this driver, so do that as well.

Signed-off-by: Thierry Reding <[e-mail]>

drivers/gpu/drm/tegra/dc.c

···
 	if (!IS_ERR(primary))
 		drm_plane_cleanup(primary);
 
-	if (group && tegra->domain) {
-		iommu_detach_group(tegra->domain, group);
+	if (group && dc->domain) {
+		if (group == tegra->group) {
+			iommu_detach_group(dc->domain, group);
+			tegra->group = NULL;
+		}
+
 		dc->domain = NULL;
 	}
 
···
 
 static int tegra_dc_exit(struct host1x_client *client)
 {
+	struct drm_device *drm = dev_get_drvdata(client->parent);
 	struct iommu_group *group = iommu_group_get(client->dev);
 	struct tegra_dc *dc = host1x_client_to_dc(client);
+	struct tegra_drm *tegra = drm->dev_private;
 	int err;
 
 	devm_free_irq(dc->dev, dc->irq, dc);
···
 	}
 
 	if (group && dc->domain) {
-		iommu_detach_group(dc->domain, group);
+		if (group == tegra->group) {
+			iommu_detach_group(dc->domain, group);
+			tegra->group = NULL;
+		}
+
 		dc->domain = NULL;
 	}