drm: sun4i: Add support for suspending the display driver

Shut down the display engine during suspend.

Signed-off-by: Ondrej Jirman <megous@megous.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20191029112846.3604925-1-megous@megous.com