FWIW, I found a thread reporting the same behaviour (re HDMI) for a similar Dell laptop model.

Just install OpenSuse Leap 42.2 on my new laptop (Dell Inspiron 15 7567 Gaming). I am unable to make my screen work through HDMI.
I also found out that the machine is made in a little funky way. The laptop screen is using the Intel card and the HDMI port is on the NVIDIA card. It appear that opensuse or the driver has hard time handling a situation like this.
So, probably sums it up.