drm/xe/migrate: fix copy direction in access_memory

After we do the modification on the host side, ensure we write the
result back to VRAM and not the other way around, otherwise the
modification will be lost if treated like a read.

Fixes: 270172f6 ("drm/xe: Update xe_ttm_access_memory to use GPU for non-visible access")
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Link: https://lore.kernel.org/r/20250710134128.800756-2-matthew.auld@intel.com