drm/xe: Support copying of data between system memory bos

Modify the xe_migrate_copy() function somewhat to explicitly allow
copying of data between two buffer objects including system memory
buffer objects. Update the migrate test accordingly.

v2:
- Check that buffer object sizes match when copying (Matthew Auld)

Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>