You are one of the lucky ones! Normally, Intel Quick Sync can’t engage in Windows without seeing an active monitor. The easiest way to fool a system like that is to just buy a dummy plug from Amazon, with either HDMI or Displayport as needed. I can personally confirm I ran into this problem with acceleration. A search for dummy plug on the forums will bring up a decent amount of history, but here’s one from earlier this year that spells out some details:
