drm/mediatek: Declare Z Position for all planes

MediaTek SoCs support multiple planes, one of which is the primary
and all the others are overlays (and CURSOR is the last overlay).

In all currently supported SoCs, the Z order of the overlays can't
be changed with any fast muxing action, and can only be changed by
swapping the contents of the entire register set of one overlay
with the other to internally reorder the layer properties, which
is indeed feasible, but probably more expensive than desired.

Declare the Z position for all planes with an immutable property
at least for now, so that the userspace can take its decisions
accordingly.

Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: Fei Shao <fshao@chromium.org>
Acked-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: CK Hu <ck.hu@mediatek.com>
Link: https://patchwork.kernel.org/project/dri-devel/patch/20240718082507.216764-1-angelogioacchino.delregno@collabora.com/


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