drm/xe/pm: Add vram_d3cold_threshold for d3cold capable device

drm/xe/pm: Add vram_d3cold_threshold for d3cold capable device

Do not register vram_d3cold_threshold device sysfs universally
for each gfx device, only register sysfs and set the threshold
value for d3cold capable devices.

Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Anshuman Gupta <anshuman.gupta@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://lore.kernel.org/all/20230802070449.2426563-1-anshuman.gupta@intel.com/
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>