Fix undefined macros in "Tuners Positions" in DTL main screen

There is a macro called PREFIX which is undefined in section "Tuners Positions", in DTL OPI main screen. This can be acknowledge by a series of runtime errors like the one below:

2022-08-22 14:07:30 WARNING [org.csstudio.display.builder.runtime] 
Widget 'Text Entry_11' (textentry) and Script 'JythonScript textentry:Text Entry_11:DisabledInCloseLoop.rule.py':
pv2 '$(PREFIX):EMR-SM-003:CtrlPID-Enb' is not fully resolved: $(PREFIX):EMR-SM-003:CtrlPID-Enb

The macro PREFIX is used by the 3 labels called Tuner 1, 2 and 3, by their Rules property. But this macro is undefined, thus we have the error above.