Hi,
I bought a Inspiron G3 15-3590 and had the same problem, luckily it came with bios 1.8.0, this is how I solved it.
I went to: Device Manager > Display Adapters. Right click on Intel Integrated Graphics > Disable device.
For my Inspiron, this setup will define Nvidia as default, but you can always enable Intel graphics again.
XPS 15-9570 users =
1. Update the BIOS to 1.15.1
2. After reboot, install the Dell Intel Chipset driver 10.1.17541.8066 and reboot
3. Install the Dell Intel GPU driver 26.20.100.7262 and reboot
4. Install the Dell Nvidia driver 26.21.14.4145 and reboot
<ADMIN NOTE: Broken link has been removed from this post by Dell>
Hope this helps all the way from Brazil.