drm/etnaviv: Add a helper to get the first available GPU device node

This patch make the code in the etnaviv_pdev_probe() less twisted, and it
also make it easier to drop the reference to device node after finished.
Before apply this patch, there is no call to of_node_put() when done. We
should call of_node_put() when done.

Signed-off-by: Sui Jingfeng <suijingfeng@loongson.cn>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>