drm/mediatek: mtk_dpi: Add local helpers for bus format parameters

drm/mediatek: mtk_dpi: Add local helpers for bus format parameters

In preparation for adding support for additional color formats,
add local helpers to map media bus format parameters to this
driver's bit_num, channel_swap and color_format.

This commit brings no functional changes.

Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: CK Hu <ck.hu@mediatek.com>
Link: https://patchwork.kernel.org/project/dri-devel/patch/20250409131306.108635-3-angelogioacchino.delregno@collabora.com/
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>