drm/mediatek: Check if fb is null

[ Upstream commit b1d685b6467ac0d98fc63989f71b4ca9186be5d4 ]

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

Fixes: b6b1bb980ec4 ("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>
Signed-off-by: Sasha Levin <sashal@kernel.org>
1 file changed