drm/mediatek: Add encoder_index interface for mtk_ddp_comp_funcs

To support dynamic connector selection function, each ddp_comp need to
get their encoder_index to identify which connector should be selected.

Add encoder_index interface for mtk_ddp_comp_funcs to get the encoder
identifier by drm_encoder_index().
Then drm driver will call mtk_ddp_comp_encoder_index_set() to store the
encoder_index to each ddp_comp in connector routes.

Signed-off-by: Jason-JH.Lin <jason-jh.lin@mediatek.com>
Reviewed-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/20231004024013.18956-5-jason-jh.lin@mediatek.com/


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