drm/sun4i: Move interlace related code in DE2

There is no point having code which sets interlace mode of mixer in
channel related function. Interlace mode will only change when CRTC
state will change, so let's move it to the block which is executed only
when primary plane state is changed.

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/20171201060550.10392-16-jernej.skrabec@siol.net