Hi, i Found a solution. There is a soldring point for a sensor (near the home button or on left hand side when ipad dizitizer front side is down). when you used after market dizitizer on that there is no sensor there is blank space. so what have to do now if you can't put sensor over there, just wet your fingre little bit and clean then touch on blank soldering point, when you touch there your device will wake and sleep by touching when its wake by touching there then just go to setting > screen brightness > then only option will come for (power button) (Lock/Unlock) by on/off that your ipad mini power button will work start again as it was working before. but don't update it without backup other wise you have to do this again. Thanks, If you don't understand anything by this post you can contact me or ask me here.
Thanks
-Lovejeet Singh.