video: sunxi: de2: fix SimpleFB node creation when HDMI not initialized

When HDMI is not initialized (e.g. no monitor is plugged), the current
SimpleFB code will still create a broken SimpleFB node.

Detect whether HDMI is initialized when creating SimpleFB node.

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