drm/mediatek: Check if fb is null

It's possible that state->base.fb is null. Add a check before access its
format.

Fixes: b6b1bb98 ("drm/mediatek: Turn off Alpha bit when plane format has no alpha")
Signed-off-by: Yongqiang Niu <yongqiang.niu@mediatek.com>
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>

drivers/gpu/drm/mediatek/mtk_disp_ovl.c

@@ -291,7 +291,7 @@ void mtk_ovl_layer_config(struct device *dev, unsigned int idx,
 	}
 
 	con = ovl_fmt_convert(ovl, fmt);
-	if (state->base.fb->format->has_alpha)
+	if (state->base.fb && state->base.fb->format->has_alpha)
 		con |= OVL_CON_AEN | OVL_CON_ALPHA;
 
 	if (pending->rotation & DRM_MODE_REFLECT_Y) {