Hi,
I have exactly the same problem as another dell U2414H owner about a year ago.
In this case a dell U2414H and a hp dc7900 ultra slim with DP (with onboard intel gpu chipset). This pc has also a vga port that works ok (when i connect it with a vga monitor).
When I turn on my PC, the monitors remain in power save mode until the windows login screen appears. After that point they function with no issue whatsoever. If I hit the bios key, the PC will enter bios, but my screen just stay in power save, so I'm stuck until I reboot manually.
(Same problem as Posted by hyperblau
In his case a dvi to hdmi solved the problem. I already buy an dp to hdmi cable but with no success.
(The only long shot a can think is an dp to dvi cable and and dvi to hdmi in chain.)