drm/sun4i: Set blending mode for all channels (DE2)

BSP driver always sets blend mode for all channels, no matter if they
are really used or not. Do the same here.

The exact meaning of the value is not exactly known, but BSP driver
mentions "SRC OVER" and by digging through code some more info can be
found.

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/20171201060550.10392-8-jernej.skrabec@siol.net