This was broken since cca87ccc75
The problem was that as this popover was defined in the C-code
for the Graph Editor, the relevant panel never got added to the
WM_paneltype registry, since only Python defined panels get
WM_paneltype_add() called when they are defined.
Since the majority of panels defined for regions are only used there,
a more localised fix was applied here by only adding the driver popover
to the global list manually.
37ffd95594