drm/nouveau/pm: prevent freeing the wrong engine context

This fixes a crash when multiple PM engine contexts are created.
Signed-off-by: NSamuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: NMartin Peres <martin.peres@free.fr>
Signed-off-by: NBen Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/nvkm/engine/pm/base.c

@@ -332,9 +332,12 @@ static void
 nvkm_perfctx_dtor(struct nvkm_object *object)
 {
 	struct nvkm_pm *ppm = (void *)object->engine;
+	struct nvkm_perfctx *ctx = (void *)object;
+
 	mutex_lock(&nv_subdev(ppm)->mutex);
-	nvkm_engctx_destroy(&ppm->context->base);
-	ppm->context = NULL;
+	nvkm_engctx_destroy(&ctx->base);
+	if (ppm->context == ctx)
+		ppm->context = NULL;
 	mutex_unlock(&nv_subdev(ppm)->mutex);
 }