drm/mediatek: dpi: Add input_2pixel config support

The input pixel per iteration could be different, so we add a new config
"input_2pixel" to control this.

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/20220705102530.1344-4-rex-bc.chen@mediatek.com/


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