drm/mediatek: plane: Remove plane zpos/index

It is not actually useful to a mtk plane to know its zpos/index, so just
remove this field.

This let's completely remove struct mtk_drm_plane in a follow up patch.

Signed-off-by: Daniel Kurtz <djkurtz@chromium.org>
Signed-off-by: Bibby Hsieh <bibby.hsieh@mediatek.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/1470279597-60453-3-git-send-email-bibby.hsieh@mediatek.com