drm/xe: Move d3cold_allowed decision all together.

And let's use the VRAM threshold to keep d3cold temporarily disabled.

With this we have the ability to run D3Cold experiments just by
touching the vram_d3cold_threshold sysfs entry.

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