drm/i915: remove bogus mutex_unlock from error-path

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>

drivers/gpu/drm/i915/intel_pm.c

@@ -2289,7 +2289,6 @@ err_unpin:
 	i915_gem_object_unpin(ctx);
 err_unref:
 	drm_gem_object_unreference(&ctx->base);
-	mutex_unlock(&dev->struct_mutex);
 	return NULL;
 }