media: platform: mtk-mdp3: Fix return value check in mdp_probe()

In case of error, the function mtk_mutex_get()
returns ERR_PTR() and never returns NULL. The NULL test in the
return value check should be replaced with IS_ERR().
And also fix the err_free_mutex case.

Fixes: 61890cca ("media: platform: mtk-mdp3: add MediaTek MDP3 driver")
Signed-off-by: Qiheng Lin <linqiheng@huawei.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>