drm/layerscape: fix handling fsl_dcu_drm_plane_index result

The function can return negative value.

The problem has been detected using proposed semantic patch
scripts/coccinelle/tests/unsigned_lesser_than_zero.cocci [1].

[1]: http://permalink.gmane.org/gmane.linux.kernel/2038576

Signed-off-by: Andrzej Hajda <[e-mail]>
Signed-off-by: Dave Airlie <[e-mail]>

drivers/gpu/drm/fsl-dcu/fsl_dcu_drm_plane.c

···
 					     struct drm_plane_state *old_state)
 {
 	struct fsl_dcu_drm_device *fsl_dev = plane->dev->dev_private;
-	unsigned int index, value, ret;
+	unsigned int value;
+	int index, ret;
 
 	index = fsl_dcu_drm_plane_index(plane);
 	if (index < 0)