Revert "drm/etnaviv: trick drm_mm into giving out a low IOVA"

Now that commandstreams are handled through the cmdbuf suballocator
the workaround to make the IOVA games work is not needed anymore.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>