Have you also looked at the Windows Display Settings in Control Panel? Would there be any options listed there as 1080p?
Another thing, can you try a DVI-to-HDMI cable? DVI connected to your RX 470 and the HDMI to your TV. I have read some issues before that using HDMI-to-HDMI on TVs results in some display problems for Windows (not being able to get the correct resolution as well as the refresh rate). Also, as another resort, can you also try a different cable (just in case that maybe the HDMI cable you are currently using is defective?).