That's odd that that problem has returned with newer cards, it used to be a problem a year or so back with older catalyst drivers, but then it was fixed in the driver end, and we've not had to worry about it...
But the fix then was to use monitor "dummy plugs" basically these were a DVI to VGA adapter with some resistors stuffed in across the outputs such that it thought there was an analog monitor connected. These are rather simple to make or can be ordered online from some of the crypto mining hardware specialists. (I'm thinking memory dealers and winnipeg technology and computer renewal at the moment) But there was some suppliers in the classified here, but they may have stopped offering them recently due to lack of need... although if the newer cards are going to be a bugger again, I suspect they will be very willing to sell them again.
Here's one page on how to make one...
http://www.geeks3d.com/20091230/vga-hack-how-to-make-a-vga-dummy-plug/