drm/sun4i: Add support for A64 display engine

Display Engine(DE2) in Allwinner A64 has two mixers and tcons.

The routing for mixer0 is through tcon0 and connected to
LVDS/RGB/MIPI-DSI controller.

The routing for mixer1 is through tcon1 and connected to HDMI.

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180904044053.15425-6-icenowy@aosc.io