drm/vmwgfx: Reinstate the legacy display system dirty callback

It somehow got lost in a rewrite.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Sinclair Yeh <syeh@vmware.com>
diff --git a/drivers/gpu/drm/vmwgfx/vmwgfx_kms.c b/drivers/gpu/drm/vmwgfx/vmwgfx_kms.c
index 06ff7c8..ae87e7e 100644
--- a/drivers/gpu/drm/vmwgfx/vmwgfx_kms.c
+++ b/drivers/gpu/drm/vmwgfx/vmwgfx_kms.c
@@ -632,10 +632,13 @@
 						  true,
 						  NULL);
 		break;
+	case vmw_du_legacy:
+		ret = vmw_kms_ldu_do_dmabuf_dirty(dev_priv, &vfbd->base, 0, 0,
+						  clips, num_clips, increment);
+		break;
 	default:
-		ret = -ENOSYS;
-		WARN_ONCE(true,
-			  "Dirty called with invalid display system.\n");
+		ret = -EINVAL;
+		WARN_ONCE(true, "Dirty called with invalid display system.\n");
 		break;
 	}