drm/stm: ltdc: fix warnings in ltdc_plane_create()

"make C=1" returns 2 warnings in ltdc_plane_create()
("Using plain integer as NULL pointer"). This patch
fixes them.
Signed-off-by: Philippe Cornu <philippe.cornu@st.com>
Reviewed-by: Yannick Fertré <yannick.fertre@st.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180419132804.8317-1-philippe.cornu@st.com

drivers/gpu/drm/stm/ltdc.c

@@ -860,13 +860,13 @@ static struct drm_plane *ltdc_plane_create(struct drm_device *ddev,
 
 	plane = devm_kzalloc(dev, sizeof(*plane), GFP_KERNEL);
 	if (!plane)
-		return 0;
+		return NULL;
 
 	ret = drm_universal_plane_init(ddev, plane, possible_crtcs,
 				       &ltdc_plane_funcs, formats, nb_fmt,
 				       NULL, type, NULL);
 	if (ret < 0)
-		return 0;
+		return NULL;
 
 	drm_plane_helper_add(plane, &ltdc_plane_helper_funcs);