drm/sun4i: Move channel size related code in DE2

Channel size should be set every time plane is changed, not only when
primary plane changes. Current code works only because only one
(primary) plane is supported at the moment.

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/20171201060550.10392-15-jernej.skrabec@siol.net