drm/etnaviv: iommuv1: remove map_lock

It wasn't protecting anything, as the single word writes used to
set up or tear down a translation are already inherently atomic,
so the spinlock is pure overhead.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
Reviewed-By: Wladimir J. van der Laan <laanwj@gmail.com>