drm/etnaviv: reap idle softpin mappings when necessary

Right now the only point where softpin mappings get removed from the
MMU context is when the mapped GEM object is destroyed. However,
userspace might want to reuse that address space before the object
is destroyed, which is a valid usage, as long as all mapping in that
region of the address space are no longer used by any GPU jobs.

Implement reaping of idle MMU mappings that would otherwise
prevent the insertion of a softpin mapping.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Tested-by: Guido Günther <agx@sigxcpu.org>
Acked-by: Guido Günther <agx@sigxcpu.org>
Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>