If autoprobing outputs fails - probably because the monitor sits behind a KVM - neither the xorg driver nor the nouveau kernel module can force-enable an output.
Details are as possible:
I have here an old GForce FX 5200 card whose DVI-I ouptut is connected to a monitor behind a KVM switch (an ATEN CS22D). If I turn the system on with the monitor switched away, then switch back after the system booted up, the monitor remains blank. The system is running a custom 4.1.15 kernel with xorg 7.7.
I tried the following:
*) Downloaded the edid of the monitor and placed it in lib/firmware. Then enforced loading the custom edid with the kernel parameter drm_kms_helper.load_edid=edid/edid.bin. The kernel log indicates that the custom edid is loaded correctly when the monitor is connected, though the overall problem does not change. The monitor still remains blank when disconnected during bootstrap and reconnected after boot-up.
According to the system, the monitor is connected to the output DVI-I-1, and I verified this with both the kernel log and xrandr.
*) Inserted the kernel parameter video=DVI-I-1:e to force-enable the output. The result of this is that the video output now remains always blank, regardless of whether the monitor is connected during bootstrap or not. Thus, the parameter does quite the opposite of what it should do. It rather force-disables the output.