drm/sun4i: Add support for DE2 VI planes

This commit adds basic support for VI planes. They are meant for video
overlay and because of that they support YUV formats too. However, using
YUV format is not straightforward, so only RGB formats are supported for
now.

Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171201060550.10392-20-jernej.skrabec@siol.net