Sorry to reopen an existing thread but....
Man, I wish I found this a few months ago (ironic it was on this forum) I have been running libreelec for a while and have ambilight using hyperhdr. Unfortunately kodi wouldn't take the HDMI source as the primary display all the time and tried to output through the USB grabber, it thought was a display and would display a black screen. This resulted in me running libreelec on one NUC and hyperhdr/ubuntu on another...This was obviously less than desirable.
I tried all the above which was recommended by others on other forums but nothing worked until just appending a video parameters to grub solved it. (Using an edid file did work but it broke HDR capabilities.)
Thank you kevink
Also I had a similar issue a year ago where I had a DP -> HDMI cable going from libreelec NUC to a receiver. This was in place as the receiver only supported 7.1 channel TrueHD etc audio on specific HDMI ports (due to the limitations of ARC). I ended up settling for 5.1 audio due to this.
I'm 99% sure when I plug the dp cable back in and send audio over that from kodi this will solve that issue too. Winning.
Hopefully that helps others and mods may consider this post as sticky. If not delete it.
Here is the explanation of grub video(and others) parameters: http://distro.ibiblio.org/fatdog/web/faqs/boot-options.html