[BUGFIX] FAL viewhelper does not work in backend context

The method \TYPO3CMS\Core\Resource\FileRepository->findByRelation()
does not work in TYPO3 6.2.4 anymore in backend context.

However this patch uses now the ->findFileReferenceByUid() method and
get the relations manually from database.

Resolves: #60706
Change-Id: I7808b6b7ed1c95853c873dd2ebfd8572ab365cd5
Reviewed-on: http://review.typo3.org/32569
Reviewed-by: Armin Ruediger Vieweg <armin@v.ieweg.de>
Tested-by: Armin Ruediger Vieweg <armin@v.ieweg.de>