[BUGFIX] FormEngine: Only evaluate Flexform display conditions once

Avoid evaluating the display conditions for fields on a Flexform sheet
more than once. Multiple evaluations of the same conditions can lead to
wrong result due to fields having been removed by the first evaluation.

Resolves: #70314
Releases: master
Change-Id: I412a8cc2acf67e2a72de1f37a0f8038110ca4c2e
Reviewed-on: http://review.typo3.org/43744


Reviewed-by: Morton Jonuschat <m.jonuschat@mojocode.de>
Tested-by: Morton Jonuschat <m.jonuschat@mojocode.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>