video: sunxi: dw-hdmi: read source_id later

There is no real need to read source_id at probe time. It also doesn't
make sense to store it in driver private data since it's already stored
in class platform data. While this looks like cleanup (and it is), it's
also important for DE2 driver rework because this info will be filled
later (after probe is already executed).

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>