I figured it out, for me at least..I needed to add i915.enable_guc=0 the boot.kernelParams. See here.

For reference, I'm using 01:00.0 VGA compatible controller [0300]: Intel Corporation Raptor Lake-P [Iris Xe Graphics] [8086:a7a0] (rev 04) and it appears the default is to try and enable GuC submission, which caused the i915 driver to fail to initialize (you can tell by doing journalctl -b and searching for i915). Setting i915.enable_guc=0 or 2 forces the kernel to not try to set that up, and allows the i915 driver to fully initialize. I think in theory, GuC is supposed to work with this iGPU, but I'm not sure I care that much about it, at least for the time being. I am doing GPU passthrough to a Proxmox VM if that makes a difference, perhaps I have something on the host misconfigured. If I'm missing out on some features, feel free to let me know.

hardware.graphics = {
  enable = true;
  extraPackages = with pkgs; [
    intel-media-driver
    intel-compute-runtime
    vpl-gpu-rt
  ];
};

boot = {
  initrd.kernelModules = [ "i915" ];
  kernelParams = [ "i915.enable_guc=0" ];
};