drm/mediatek: Ensure the cursor plane is on top of other overlays

Currently the cursor is placed on the first overlay plane, which means
it will be at the bottom of the stack when the hw does the compositing
with anything other than primary plane. Since mtk doesn't support plane
zpos, change the cursor location to the top-most plane.

Signed-off-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: CK Hu <ck.hu@mediatek.com>