sunxi: display: Fix gpio handling to work with the driver-model

The driver-model gpio functions may return another value then -1 as error,
make the sunxi display code properly handle this.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Acked-by: Ian Campbell <ijc@hellion.org.uk>