drm/sun4i: Rename DE2 RGB format macros

Current RGB formats macros are actually not specific to UI planes.
Rename it to something more universal and introduce shift macro.

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/20171201060550.10392-3-jernej.skrabec@siol.net