drm/xe/pm: Add vram_d3cold_threshold Sysfs

Add per pci device vram_d3cold_threshold Sysfs to
control the d3cold allowed knob.
Adding a d3cold structure embedded in xe_device to encapsulate
d3cold related stuff.

v2:
- Check total vram before initializing default threshold. [Riana]
- Add static scope to vram_d3cold_threshold DEVICE_ATTR. [Riana]
v3:
- Fixed cosmetics review comment. [Riana]
- Fixed CI Hook failures.
- Used drmm_mutex_init().
v4:
- Fixed kernel-doc warnings.
v5:
- Added doc explaining need for the device sysfs. [Rodrigo]
- Removed TODO comment.

Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Anshuman Gupta <anshuman.gupta@intel.com>
Reviewed-by: Riana Tauro <riana.tauro@intel.com>
Acked-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230718080703.239343-4-anshuman.gupta@intel.com
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>