Viewsonic HDMI

Just bought a ViewSonic vx2758-c-mh 1080p 144hz display. The specs claim that it only supports 144hz over HDMI, and I am using the supplied HDMI cable from the box, but highest refresh rate I can achieve is 120hz. Graphics card is an r9 380 (which has HDMI 1.4), and I've tried setting 144hz...