drm/rockchip: only call drm_fb_helper_hotplug_event if fb_helper present

Add a check for the presence of fb_helper to rockchip_drm_output_poll_changed()
to only call drm_fb_helper_hotplug_event if there is actually a fb_helper
available. Without this check I see NULL pointer dereferences when the
hdmi hotplug irq fires before the fb_helper got initialized.
Signed-off-by: NHeiko Stuebner <heiko@sntech.de>

drivers/gpu/drm/rockchip/rockchip_drm_fb.c

@@ -162,7 +162,8 @@ static void rockchip_drm_output_poll_changed(struct drm_device *dev)
 	struct rockchip_drm_private *private = dev->dev_private;
 	struct drm_fb_helper *fb_helper = &private->fbdev_helper;
 
-	drm_fb_helper_hotplug_event(fb_helper);
+	if (fb_helper)
+		drm_fb_helper_hotplug_event(fb_helper);
 }
 
 static const struct drm_mode_config_funcs rockchip_drm_mode_config_funcs = {