ARM: sun5i: a13-olinuxino: Enable VGA bridge

Now that we have support for the VGA bridges using our DRM driver, enable
the display engine for the Olimex A13-Olinuxino.

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