ARM: dts: sun6i: hummingbird: Enable display engine again

Now that we disable the display engine by default, we need to re-enable
it for the Hummingbird A31, which already had its display pipeline
enabled.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>