drm/i915/dgfx: Enable d3cold at s2idle

drm/i915/dgfx: Enable d3cold at s2idle

System wide suspend already has support for lmem save/restore during
suspend therefore enabling d3cold for s2idle and keepng it disable for
runtime PM.(Refer below commit for d3cold runtime PM disable justification)
'commit 66eb93e71a7a ("drm/i915/dgfx: Keep PCI autosuspend control
'on' by default on all dGPU")'

It will reduce the DG2 Card power consumption to ~0 Watt
for s2idle power KPI.

v2:
- Added "Cc: stable@vger.kernel.org".

Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/8755
Cc: stable@vger.kernel.org
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Anshuman Gupta <anshuman.gupta@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Tested-by: Aaron Ma <aaron.ma@canonical.com>
Tested-by: Jianshui Yu <Jianshui.yu@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230816125216.1722002-1-anshuman.gupta@intel.com