drm/mediatek: Fix dereference before null check

Null-checking state suggests that it may be null, but it has already
been dereferenced on drm_atomic_get_new_plane_state(state, plane).

The parameter state will never be NULL currently, so just remove the
state is NULL flow in this function.

Fixes: 5ddb0bd4 ("drm/atomic: Pass the full state to planes async atomic check and update")
Signed-off-by: Jason-JH.Lin <jason-jh.lin@mediatek.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: CK Hu <ck.hu@mediatek.com>
Reviewed-by: Alexandre Mergnat <amergnat@baylibre.com>
Link: https://patchwork.kernel.org/project/dri-devel/patch/20230714094908.13087-5-jason-jh.lin@mediatek.com/


Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>