drm/xe: Update comment on why d3cold is still blocked.

The main issue with buddy allocator was fixed, but then
we ended up on other issues, so we need to step back and rethink
our strategy with D3cold. So, let's update the comment with a
todo list so we don't get tempted in removing it before we are
really ready.

Cc: Matthew Auld <matthew.auld@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Riana Tauro <riana.tauro@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>