drm/sun4i: Explain color macro in DE2 driver

Color attribute have same format troughout the whole driver.

Rename macro, add comment with simple explanation and remove redundant
definitions.

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/20171201060550.10392-7-jernej.skrabec@siol.net