drm/etnaviv: convert user fence tracking to XArray

This simplifies the driver code a bit, as XArray already provides
internal locking. IDRs are implemented using XArrays anyways, so
this drops one level of unneeded abstraction.

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