drm/xe: Only set PCI d3cold_allowed when we are really allowing.

First of all it was strange to see:
if (allowed) {
...
} else {
   D3COLD_ENABLE
}

But besides this misalignment, let's also use the pci
d3cold_allowed useful to us and know that we are not really
allowing d3cold.

Cc: Anshuman Gupta <anshuman.gupta@intel.com>
Reviewed-by: Anshuman Gupta <anshuman.gupta@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>