drm/xe/xe_migrate: Use NULL 1G PTE mapped at 255GiB VA for ccs clear

Get rid of the cleared bo, instead use null 1G PTE mapped at 255GiB
offset, this can be used for both dgfx and igfx.

v2:
 - Remove xe_migrate::cleared_bo.
 - Add a comment for NULL mapping.(Thomas)

Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Signed-off-by: Himal Prasad Ghimiray <himal.prasad.ghimiray@intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>