drm/etnaviv: add missing MMU context put when reaping MMU mapping

When we forcefully evict a mapping from the the address space and thus the
MMU context, the MMU context is leaked, as the mapping no longer points to
it, so it doesn't get freed when the GEM object is destroyed. Add the
mssing context put to fix the leak.

Cc: stable@vger.kernel.org # 5.4
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Tested-by: Michael Walle <michael@walle.cc>
Tested-by: Marek Vasut <marex@denx.de>
Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>