drm/etnaviv: fix MMU context leak on GPU reset

commit f978a5302f5566480c58ffae64a16d34456801bd upstream.

After a reset the GPU is no longer using the MMU context and may be
restarted with a different context. While the mmu_state proeprly was
cleared, the context wasn't unreferenced, leading to a memory leak.

Cc: stable@vger.kernel.org # 5.4
Reported-by: Michael Walle <michael@walle.cc>
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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>