Revert "drm/xe: Enable d3cold support for dgfx"

This reverts commit 7c1c3d38.

Unfortunately we are not ready for this step yet.

We need to rethink our strategy on D3cold:

1. rewrite the VRAM save / restore to avoid buffer object locks
2. block D3cold if we have a big amount of device memory in use in order
   to reduce the latency.
3. at resume, detect if we really lost power and avoid memory restoration
   if we were only up to d3cold

Reference: #256


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>