[BUGFIX] MM relations are not correctly resolved in Backend view

DataPreprocessor is taking care of preparing data for the form
view processed in the FormEngine. However, MM relations are not
correctly resolved in a workspace context.

The method DataPreprocessor::getDataIdList() is used for a mixed
kind of relations there, without properly applying the differences
for MM records. The rule is, to always use the live default id,
except for MM relations - use the specific workspace value.

Resolves: #58735
Releases: 6.2
Change-Id: I256969adb46bfea80681160e2901387a8c7c9a7d
Reviewed-on: https://review.typo3.org/30037
Reviewed-by: Georg Ringer
Tested-by: Georg Ringer
Reviewed-by: Markus Klein
Reviewed-by: Helmut Hummel
Tested-by: Helmut Hummel