[BUGFIX] displayCondition on FlexForm Sheets broken

Due to the usage of an undefined variable, the value of
an field within the flexform cannot resolved correctly.
As an result any displayCondition of an FlexForm sheet looking
at a field value (FIELD:*:=:value) evaluates to false.

Change-Id: I31bf3af8b61981d7fde9e3d0f1c6dbc01118d14a
Fixes: #35436
Releases: 6.0,4.7
Reviewed-on: http://review.typo3.org/10107
Reviewed-by: Jigal van Hemert
Tested-by: Jigal van Hemert
Reviewed-by: Dominik Mathern
Tested-by: Dominik Mathern
Reviewed-by: Steffen Ritter
Tested-by: Steffen Ritter
Reviewed-on: http://review.typo3.org/10221