drm/etnaviv: fix MMU context leak on GPU reset

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>

drivers/gpu/drm/etnaviv/etnaviv_gpu.c

@@ -571,6 +571,8 @@ static int etnaviv_hw_reset(struct etnaviv_gpu *gpu)
 
 	gpu->fe_running = false;
 	gpu->exec_state = -1;
+	if (gpu->mmu_context)
+		etnaviv_iommu_context_put(gpu->mmu_context);
 	gpu->mmu_context = NULL;
 
 	return 0;