No audio or video if machine booted without display on; can this be forced?

I have a Beelink SER3 mini-pC running Debian 12, fully patched. This machine is based on the AMD Ryzen 7 3750H with Radeon Vega Mobile Gfx. The HDMI port is plugged into a Denon AVR receiver, and this receiver is plugged into a 4K TV. This works well. The X server starts up in 4K, and pulseaudio happily sends audio via the HDMI port, which the receiver can play through speakers. The problem is if the machine is turned on while the TV is off. In this case it doesn't detect any ouput; pulseaudio goes to a null-sink and even if I turn on the TV later I don't get a display. A quick reboot with the TV on and everything is OK again. Without the TV I see an extra line in the boot messages

kernel: [    4.095325] amdgpu 0000:04:00.0: [drm] Cannot find any crtc or sizes

That's pretty much the only difference in "amdgpu" boot messages from the kernel. Is there a way of forcing the kernel to believe there's a display connected at bootuo, even if there isn't? Or convincing pulseaudio to work (it just needs the AVR, not the TV) and the X server to detect when the TV is turned on?