drm/exynos: add drm_iommu_attach_device_if_possible()

Every CRTC drivers in Exynos DRM implements the code which checks
whether IOMMU is supported or not, and if supported enable it.
Making new helper for it generalize each CRTC drivers.

Signed-off-by: Hyungwon Hwang <human.hwang@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>
diff --git a/drivers/gpu/drm/exynos/exynos_mixer.c b/drivers/gpu/drm/exynos/exynos_mixer.c
index f570687..cae98db 100644
--- a/drivers/gpu/drm/exynos/exynos_mixer.c
+++ b/drivers/gpu/drm/exynos/exynos_mixer.c
@@ -882,10 +882,12 @@
 		}
 	}
 
-	if (!is_drm_iommu_supported(mixer_ctx->drm_dev))
-		return 0;
+	ret = drm_iommu_attach_device_if_possible(mixer_ctx->crtc, drm_dev,
+								mixer_ctx->dev);
+	if (ret)
+		priv->pipe--;
 
-	return drm_iommu_attach_device(mixer_ctx->drm_dev, mixer_ctx->dev);
+	return ret;
 }
 
 static void mixer_ctx_remove(struct mixer_context *mixer_ctx)