I was having the same problem.  Uninstalled and reinstalled the latest display drivers.  Nothing.  Made sure Windows 10 was updated.  Nothing.  Restarted the computer several times.  Nothing.  When I bumped into this post, I realized that you and I are using the exact same monitor.  Too much of a coincidence.  I knew it had to have something to do with HDCP or the Displayport connection.  Power off and unplug the power cable from your monitor for 10 seconds.  Yes you have to unplug it.  Then you'll get your resolution options back.  Cheers.
I upgraded the OS to Windows 10. Windows 8.1 was operating fine. After the upgrade, my resolution is 640X480. I can't change it anywhere. Device Manager initially showed the monitor as a Generic Non PNP monitor. I was able to change it to the U2412M with DisplayPort. It says it has the most recent drivers. If I try to recognize it manually, it will revert back to the Generic one. When I attempt to update the Catalyst dashboard, drivers, etc. it says I have the latest there, too. However, when I attempt to change the resolution either under Windows 10 from the desktop or from the Catalyst dash, my only choice is 640X480. On the Windows desktop, the pull down menu is grayed out so I can not choose any other resolution. In the Catalyst dash, there are no other choices. Any thoughts on how to get to a usuable resolution? My computer is a year old, and all of the components are still being used on the latest 8700 SE being sold, and advertised as Windows 10 compatible.
XPS 8700 SE, AMD R9 270, Dell U2412M Monitor, 640X480 only resolution Windows 10