drm/sun4i: de2: rgb field in de2 format struct is redundant

drm_format_info structure already contains information if format is RGB
or YUV. Use that instead.

Acked-by: Maxime Ripard <mripard@kernel.org>
Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20200224173901.174016-5-jernej.skrabec@siol.net