drm/mediatek: dpi: Add pixels_per_iter config support

The quantity of output for one iteration could be different for dpi and
dp_intf. For dp_intf, it's 4 pixels for one iteration it's 1 pixel for one
iteration for dpi. Therefore, we add a new config "pixels_per_iter" to
control quantity of transferred pixels per iteration.

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/20220705102530.1344-5-rex-bc.chen@mediatek.com/


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