Force VGA output when multiple video devices exist
I want to install Fedora 22 (soon 23) on systems with multiple display options and force the output to a VGA device.
There must be other folks that have lots of old VGA displays wanting to use them with newer computers experiencing the same problem. The install goes well with output on the VGA monitor. But then when Fedora boots you get no output due to the system having more than one display device on the motherboard.
I first experienced this with an Intel Atom D2500 in a Mini-ITX fanless case. The plan was to use this dual NIC system as a firewall. It would run mostly headless other than the initial configuration and perhaps occasional configuration if I couldn't do something with a remote connection. Having lots of old VGA monitors sitting around I wanted to use one as the monitor during configuration. The install went just fine but when Fedora 22 booted I had no video. I found a reviewer on Amazon giving the hint to add to the kernel boot line "video=LVDS-1:d video=DVI-D-1:d video=DVI-D-2:d". Sure enough, that did make the video appear on the VGA port. No explanation of why this works or how one might figure it out for any arbitrary system was given.
I just got a used Dell Optiplex 790 computer that I wish to use as a media server. I would like to run headless. I connected an old VGA monitor and the install went just fine. But when Fedora 22 booted I had no video. I borrowed a new display from another system and connected the display port cable. That brought up a display on both the new monitor on display port and the old monitor on VGA. The login window appears on the display port.
I will bet that there is some collection of arguments for the boot line that would have this Dell system use only the VGA monitor -- however I do not know what this would be.
Question: How do I figure out for any given system having multiple displays, what is needed to force output to one particular display - often the VGA display? And how do I make sure that this selection remains in effect across system upgrades? It seems that this should be a well documented bit of information that is easily found as there must be other folks hitting the problem.
Thanks for assistance, explanations and pointers.