drm/xe: Move migrate to prefetch to op_lock_and_prep function

All non-binding operations in VM bind IOCTL should be in the lock and
prepare step rather than the execution step. Move prefetch to conform to
this pattern.

v2:
 - Rebase
 - New function names (Oak)
 - Update stale comment (Oak)

Cc: Oak Zeng <oak.zeng@intel.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Oak Zeng <oak.zeng@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240425045513.1913039-4-matthew.brost@intel.com