video: sunxi: de2: switch to public uclass functions

Currently DE2 driver uses functions which are defined in internal
headers. They are not meant to be used outside of uclass framework.
Switch DE2 driver to public ones. This has additional benefit that
device_probe doesn't need to be called manually.

Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>