This has been lost in the locking rework for intel_alloc_context_page:
commit 
2c34b850ee1e9f86b41706149d0954eee58757a3
Author: Ben Widawsky <ben@bwidawsk.net>
Date:   Sat Mar 19 18:14:26 2011 -0700
    drm/i915: fix ilk rc6 teardown locking
Cc: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
        i915_gem_object_unpin(ctx);
 err_unref:
        drm_gem_object_unreference(&ctx->base);
-       mutex_unlock(&dev->struct_mutex);
        return NULL;
 }