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>