drm/sun4i: Add support for BT2020 to DE3

DE3 supports 10-bit formats, so it's only naturally to also support
BT2020 encoding.

Add support for it.

Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20210110201947.3611649-4-jernej.skrabec@siol.net