drm/xe/vf: Refactor CCS save/restore to use default migration context

Previously, CCS save/restore operations created separate migration
contexts with new VM memory allocations, resulting in significant
overhead.

This commit eliminates redundant context creation reusing the default
migration context by registering new execution queues for CCS save and
restore on the existing migrate VM.

Signed-off-by: Satyanarayana K V P <satyanarayana.k.v.p@intel.com>
Suggested-by: Matthew Brost <matthew.brost@intel.com>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: John Harrison <John.C.Harrison@Intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Stuart Summers <stuart.summers@intel.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Link: https://lore.kernel.org/r/20250808073628.32745-2-satyanarayana.k.v.p@intel.com