drm/mediatek: dpi: Add kernel document for struct mtk_dpi_conf

drm/mediatek: dpi: Add kernel document for struct mtk_dpi_conf

This driver will support dp_intf and there are many configs between dpi
and dp_intf. Therefore, we will add many configs in "struct mtk_dpi_conf".
To let this structure more readable, we add this kernel doc.

Signed-off-by: Bo-Chen Chen <rex-bc.chen@mediatek.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://patchwork.kernel.org/project/linux-mediatek/patch/20220701035845.16458-4-rex-bc.chen@mediatek.com/
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>