drm/amdgpu: Add a missing lock for drm_mm_takedown

Inside amdgpu_gtt_mgr_fini add a missing lock to maintain
locking balance

Signed-off-by: Mikita Lipski <mikita.lipski@amd.com>
Reviewed-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/amdgpu/amdgpu_gtt_mgr.c

@@ -75,7 +75,7 @@ static int amdgpu_gtt_mgr_init(struct ttm_mem_type_manager *man,
 static int amdgpu_gtt_mgr_fini(struct ttm_mem_type_manager *man)
 {
 	struct amdgpu_gtt_mgr *mgr = man->priv;
-
+	spin_lock(&mgr->lock);
 	drm_mm_takedown(&mgr->mm);
 	spin_unlock(&mgr->lock);
 	kfree(mgr);