drm/xe: Don't use drm exec locking in SVM pagefaults

Only the VM dma-resv lock is needed in SVM pagefaults so
xe_vm_lock/unlock can be used instead of drm exec. Micro optimization
but should save some CPU cycles in a critical path.

Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Stuart Summers <stuart.summers@intel.com>
Link: https://lore.kernel.org/r/20250603174012.2195759-1-matthew.brost@intel.com