video: sunxi: de2: fix SimpleFB node creation when DE2 not probed

Sometimes when a monitor without EDID information is plugged, the DE2
won't be probed (because of lack of timing information), but the HDMI
node is probed, thus a SimpleFB node with invalid information will be
populated.

Also detect whether DE2 is probed when creating SimpleFB node.

Fixes: be5b96f0 ("sunxi: setup simplefb for Allwinner DE2")
Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Acked-by: Maxime Ripard <maxime.ripard@bootlin.com>
Acked-by: Jagan Teki <jagan@openedev.com>