drm/mediatek: Add crtc path enum for all_drm_priv array

Add mtk_drm_crtc_path enum for each display path.

Instead of using array index of all_drm_priv in mtk_drm_kms_init(),
mtk_drm_crtc_path enum can make code more readable.

Signed-off-by: Jason-JH.Lin <jason-jh.lin@mediatek.com>
Reviewed-by: Fei Shao <fshao@chromium.org>
Reviewed-by: CK Hu <ck.hu@mediatek.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Tested-by: Fei Shao <fshao@chromium.org>
Link: https://patchwork.kernel.org/project/dri-devel/patch/20231004024013.18956-3-jason-jh.lin@mediatek.com/


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