[BUGFIX] Use correct variable name for backend layouts

Because of using a wrong variable name the backend layouts
are not working anymore inside a content element as the items of the
colpos list are never changed

Resolves: #53304
Releases: 6.2
Change-Id: I021e4eaaf3be56ee1ce05bd9707ffd537edb5dcc
Reviewed-on: https://review.typo3.org/25182
Reviewed-by: Georg Ringer
Tested-by: Georg Ringer