Adding support to control d3cold by using vram_usages metric from
ttm resource manager.
When root port  is capable of d3cold but xe has disallowed d3cold
due to vram_usages above vram_d3ccold_threshol. It is required to
disable d3cold to avoid any resume failure because root port can
still transition to d3cold when all of pcie endpoints and
{upstream, virtual} switch ports will transition to d3hot.
Also cleaning up the TODO code comment.

v2:
- Modify d3cold.allowed in xe_pm_d3cold_allowed_toggle. [Riana]
- Cond changed (total_vram_used_mb < xe->d3cold.vram_threshold)
  according to doc comment.
v3:
- Added enum instead of true/false argument in
  d3cold_toggle(). [Rodrigo]
- Removed TODO comment. [Rodrigo]

Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Anshuman Gupta <anshuman.gupta@intel.com>
Reviewed-by: Badal Nilawar <badal.nilawar@intel.com>
Acked-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230718080703.239343-5-anshuman.gupta@intel.comSigned-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>