drm/sun4i: Add support for H3 display engine

H3 display engine has two mixers which are connected to HDMI and TV
output.

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/20180301213442.16677-8-jernej.skrabec@siol.net