drm/sun4i: Add constraints checking to DE2 driver

Since current DE2 driver doesn't know how to scale yet, add atomic check
function which checks that.

Nice side effect of that function is that populates clipped coordinates
and checks visibility of the plane. That data will be used in the
future.

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/20171201060550.10392-12-jernej.skrabec@siol.net