drm/etnaviv: move workqueue to be per GPU

While the etnaviv workqueue needs to be ordered, as we rely on work items
being executed in queuing order, this is only true for a single GPU.
Having a shared workqueue for all GPUs in the system limits concurrency
artificially.

Getting each GPU its own ordered workqueue still meets our ordering
expectations and enables retire workers to run concurrently.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>