drm/mediatek: Fix iommu fault by swapping FBs after updating plane state

drm/mediatek: Fix iommu fault by swapping FBs after updating plane state

According to the comment in drm_atomic_helper_async_commit(),
we should make sure FBs have been swapped, so that cleanups in the
new_state performs a cleanup in the old FB.

So we should move swapping FBs after calling mtk_plane_update_new_state(),
to avoid using the old FB which could be freed.

Fixes: 1a64a7aff8da ("drm/mediatek: Fix cursor plane no 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/linux-mediatek/patch/20230809125722.24112-2-jason-jh.lin@mediatek.com/
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>