drm/mediatek: Implement OF graphs support for display paths

It is impossible to add each and every possible DDP path combination
for each and every possible combination of SoC and board: right now,
this driver hardcodes configuration for 10 SoCs and this is going to
grow larger and larger, and with new hacks like the introduction of
mtk_drm_route which is anyway not enough for all final routes as the
DSI cannot be connected to MERGE if it's not a dual-DSI, or enabling
DSC preventively doesn't work if the display doesn't support it, or
others.

Since practically all display IPs in MediaTek SoCs support being
interconnected with different instances of other, or the same, IPs
or with different IPs and in different combinations, the final DDP
pipeline is effectively a board specific configuration.

Implement OF graphs support to the mediatek-drm drivers, allowing to
stop hardcoding the paths, and preventing this driver to get a huge
amount of arrays for each board and SoC combination, also paving the
way to share the same mtk_mmsys_driver_data between multiple SoCs,
making it more straightforward to add support for new chips.

Note that the OVL_ADAPTOR software component driver needs relatively
big changes in order to fully support OF Graphs (and more SoCs anyway)
and such changes will come at a later time.
As of now, the mtk_disp_ovl_adaptor driver takes the MERGE components
(for example, on mt8195, merge 1 to 4) dynamically so, even though
later updates to the ovl-adaptor driver will *not* require bindings
changes, the merge1-4 will be temporarily omitted in the graph for the
MT8195 SoC.

This means that an example graph for this SoC looks like:

mdp_rdma (0 ~ 7) -> padding (0 ~ 7) -> ethdr -> merge5

and the resulting path in this driver will be `ovl_adaptor -> merge5`

Later updates to the ovl adaptor will expand it to support more SoCs
and, in turn, to also fully support graphs.

Reviewed-by: Alexandre Mergnat <amergnat@baylibre.com>
Tested-by: Alexandre Mergnat <amergnat@baylibre.com>
Acked-by: Sui Jingfeng <sui.jingfeng@linux.dev>
Tested-by: Michael Walle <mwalle@kernel.org> # on kontron-sbc-i1200
Reviewed-by: CK Hu <ck.hu@mediatek.com>
Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://patchwork.kernel.org/project/dri-devel/patch/20241017103809.156056-4-angelogioacchino.delregno@collabora.com/
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>

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