drm/sun4i: de2: csc_mode in de2 format struct is mostly redundant

For RGB formats CSC mode is always set to none and for YUV formats
almost always set to YUV to RGB.

Add a helper function to deduce CSC mode from format.

Acked-by: Maxime Ripard <mripard@kernel.org>
Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20200224173901.174016-6-jernej.skrabec@siol.net