drm/xe: fix suspend-resume for dgfx

This stopped working now that TTM treats moving a pinned object through
ttm_bo_validate() as an error, for the general case. Add some new
routines to handle the new special casing needed for suspend-resume.

Closes: #244


Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Tested-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>