drm/etnaviv: track fences by IDR instead of seqno

This moves away from using the internal seqno as the userspace fence
reference. By moving to a generic ID, we can later replace the internal
fence by something different than the etnaviv seqno fence.

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