A few days ago NVIDIA released a new video driver which eliminated mobile GPU overclocking in an attempt to reduce GPU overheating and GPU degradation. Apparently, after a large number of complaints the famous GPU maker has changed its decision and mobile GPU overclocking will be allowed once again and likely forever.
According to NVIDIA the possibility to overclock mobile GPUs via drivers is a developers’ mistake rather than something else. Mobile GPU overclocking will be enabled in future drivers that will be out sometime in March 2015. If you do not want to wait, though, you will have to go back to GeForce 344.75 drivers.
Source: NVIDIA