Move int voiceCount out of SurgeStorage and use polydisp directly from SurgeSynthesizer class

[nuoun]

#7727 added voice parameters to the Formula modulator and for this reason the active voice counter is stored as an int in SurgeStorage here from which the voice count is updated inside the valueAt() function in FormulaModulationHelper.cpp here.

Ideally, this value should be retrieved directly from the atomic int polydisplay in the SurgeSynthesizer class similar to how it's being used by SurgeGUIEditor.cpp here, either also as an atomic int or some other type, if the same polydisp is used it should probably also be renamed as it would no longer be used to only display a value on the GUI.