[BUG] amdgpu: shutdown hangs on Lenovo ThinkPad X13 Gen 4 (Phoenix1) without amdgpu.dcdebugmask=0x10

Hi,

System information:
  - Model: Lenovo ThinkPad X13 Gen 4 (21J3SITM18)
  - BIOS: R29ET65W (1.39)
  - Kernel: 7.0.0-28-generic (Ubuntu 24.04)
  - GPU: AMD/ATI Phoenix1 [1002:15bf] (rev de) — Radeon 780M


Current workaround:
  Kernel parameter amdgpu.dcdebugmask=0x10 is required for clean shutdown.
  Without it, the system goes through the full shutdown sequence
  (poweroff.target reached, filesystems synced, SIGTERM sent), but then
  pm_power_off() hangs — screen goes black but power stays on indefinitely.
  With amdgpu.dcdebugmask=0x10 (DC_DBG_MASK_DC bit 4), shutdown works
  normally.


  Similarly, reboot also hangs with the default EFI reset method and
  requires reboot=kbd as a workaround.


Kernel command line without workaround (shutdown hangs):
  acpi=force no_console_suspend ignore_loglevel


Kernel command line with workaround (shutdown works):
  acpi=force no_console_suspend ignore_loglevel amdgpu.dcdebugmask=0x10


The issue is 100% reproducible — every shutdown without the debug mask
hangs. A DMI quirk for this model would be appreciated so the mask is
applied automatically.


Let me know if you need any additional debug information.


Thanks,
nie