jellyfin: fix Arc A380 VAAPI transcode (ASPM power gating)

Arc A380 GPU (07:00.0) becomes unreachable (MMIO returns 0xFFFFFFFF)
when PCIe ASPM powersupersave puts it into L1.1/L1.2 substates.
Both i915 and xe drivers hit the same hardware failure.

Fix: disable runtime PM for the GPU in power-tune, run after powertop
so the override sticks. Use i915 driver (xe has iHD buffer mapping
failures on this GPU/kernel 6.12 combination).

modules/power.nix

@@ -65,10 +65,14 @@
   # ASPM powersupersave: deepest PCIe link power states (L1.1/L1.2). The
   # pcie_aspm=force boot param enables ASPM, but the runtime policy defaults
   # to "default" which only uses L0s. powersupersave adds L1 substates for
-  # all downstream devices (NVMe, AHCI, NIC).
+  # all downstream devices (NVMe, AHCI, NIC). The Intel Arc A380 GPU is
+  # excluded -- it hangs on L1 substate re-entry (MMIO returns 0xFFFFFFFF).
   systemd.services.power-tune = {
     description = "Apply power-saving sysfs knobs (EPP, ASPM policy)";
-    after = [ "multi-user.target" ];
+    after = [
+      "multi-user.target"
+      "powertop.service"
+    ];
     wantedBy = [ "multi-user.target" ];
     serviceConfig = {
       Type = "oneshot";
@@ -85,6 +89,13 @@
             # PCIe ASPM policy
             aspm=/sys/module/pcie_aspm/parameters/policy
             [ -f "$aspm" ] && echo powersupersave > "$aspm"
+
+            # Intel Arc A380 (07:00.0) hangs when PCIe link enters L1
+            # substates or runtime PM suspends it. Force the device to stay
+            # active. This runs after powertop --auto-tune (which sets
+            # power/control=auto on every device) so the override sticks.
+            gpu=/sys/bus/pci/devices/0000:07:00.0/power/control
+            [ -f "$gpu" ] && echo on > "$gpu"
           '';
         }
       );