Weston does something similar
https://cgit.freedesktop.org/wayland/weston/tree/libweston/compositor-fbdev.c#n315
...and it seems that the non-primary framebuffer devices start completely off. (the screen is off on my Displayport device, and the window for the second vga card in qemu is much smaller than the primary one, and all black for my qemu vm) .
In my testing, sending the ioctl FBIOPUT_VSCREENINFO with a *changed* &varinfo allows it to wake up, and turns on the screen.
doing FBIOGET_VSCREENINFO and then FBIOPUT_VSCREENINFO in my testing does not work.
I think really the values that end up getting changed are varinfo.transp.offset and varinfo.transp.length. at least on the qemu system, but in this patch I am aligning all of them for completeness, because the drivers might do it differently for qemu