drm/i915/gem: Check for a closed context when looking up an engine

Beware that the context may already be closed as we try to lookup an
engine.

Closes: https://gitlab.freedesktop.org/drm/intel/issues/1389
Fixes: 130a95e ("drm/i915/gem: Consolidate ctx->engines[] release")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200316161447.18410-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/gem/i915_gem_context.h

@@ -192,12 +192,16 @@ i915_gem_context_unlock_engines(struct i915_gem_context *ctx)
 static inline struct intel_context *
 i915_gem_context_get_engine(struct i915_gem_context *ctx, unsigned int idx)
 {
-	struct intel_context *ce = ERR_PTR(-EINVAL);
+	struct intel_context *ce;
 
 	rcu_read_lock(); {
 		struct i915_gem_engines *e = rcu_dereference(ctx->engines);
-		if (likely(idx < e->num_engines && e->engines[idx]))
+		if (unlikely(!e)) /* context was closed! */
+			ce = ERR_PTR(-ENOENT);
+		else if (likely(idx < e->num_engines && e->engines[idx]))
 			ce = intel_context_get(e->engines[idx]);
+		else
+			ce = ERR_PTR(-EINVAL);
 	} rcu_read_unlock();
 
 	return ce;