drm/sun4i: Add compatible for H6 display engine

H6 is first Allwinner SoC which supports 10 bit colors, HDR and AFBC.

Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181104182705.18047-9-jernej.skrabec@siol.net