drm/i915/selftests: Release ctx->engine_mutex after iteration

A lock once taken must be released again.

Fixes: c31c9e82 ("drm/i915/selftests: Teach switch_to_context() to use the context")
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>
Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191022223316.12662-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/selftests/i915_gem.c

@@ -19,18 +19,22 @@ static int switch_to_context(struct i915_gem_context *ctx)
 {
 	struct i915_gem_engines_iter it;
 	struct intel_context *ce;
+	int err = 0;
 
 	for_each_gem_engine(ce, i915_gem_context_lock_engines(ctx), it) {
 		struct i915_request *rq;
 
 		rq = intel_context_create_request(ce);
-		if (IS_ERR(rq))
-			return PTR_ERR(rq);
+		if (IS_ERR(rq)) {
+			err = PTR_ERR(rq);
+			break;
+		}
 
 		i915_request_add(rq);
 	}
+	i915_gem_context_unlock_engines(ctx);
 
-	return 0;
+	return err;
 }
 
 static void trash_stolen(struct drm_i915_private *i915)