drm/etnaviv: remove cycling through MMU address space

This was useful on MMUv1 GPUs, which don't generate proper faults,
when the GPU write caches weren't fully understood and not properly
handled by the kernel driver. As this has been fixed for quite some
time, the cycling though the MMU address space needlessly spreads
out the MMU mappings.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>