etnaviv: fix gem object list corruption

commit 518417525f3652c12fb5fad6da4ade66c0072fa3 upstream.

All manipulations of the gem_object list need to be protected by
the list mutex, as GEM objects can be created and freed in parallel.
This fixes a kernel memory corruption.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>