drm/rockchip: return ERR_PTR instead of NULL

rockchip_drm_framebuffer_init is only used in one case, in
rockchip_drm_fbdev.c, where its return value is tested using IS_ERR.  To
enable propagating the reason for the error, change the definition so that
it returns an ERR_PTR value.

Problem found with the help of Coccinelle.
Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>

drivers/gpu/drm/rockchip/rockchip_drm_fb.c

@@ -213,7 +213,7 @@ rockchip_drm_framebuffer_init(struct drm_device *dev,
 
 	rockchip_fb = rockchip_fb_alloc(dev, mode_cmd, &obj, 1);
 	if (IS_ERR(rockchip_fb))
-		return NULL;
+		return ERR_CAST(rockchip_fb);
 
 	return &rockchip_fb->fb;
 }