media: platform: mtk-mdp3: add pointer checks and use devm_kfree

Fix two errors reported by smatch:
    drivers/media/platform/mediatek/mdp3/mtk-mdp3-core.c:292
        mdp_probe() error: we previously assumed 'mdp' could be null
    drivers/media/platform/mediatek/mdp3/mtk-mdp3-cmdq.c:460
        mdp_cmdq_send() error: we previously assumed 'cmd' could be null

Also, avoid warnings reported by smatch:
    drivers/media/platform/mediatek/mdp3/mtk-mdp3-comp.c:872
        mdp_comp_create() warn: passing devm_ allocated variable to kfree. 'comp'

[hverkuil: fix devm_kfree call]

Signed-off-by: Moudy Ho <moudy.ho@mediatek.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>