drm/sun4i: Add scaler configuration to DE2 mixers

No all SoCs support scaling on all channels. For example, V3s support
scaling only on VI channels. Because of that, add additional
configuration bitmask which tells which channel support scaler.

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/20171201060550.10392-21-jernej.skrabec@siol.net