QScatterDataItem/QSurfaceDataItem: ensure initialization of d_ptr

The copy ctor did not initialize d_ptr. it called operator=, which
called createExtraData, which had a conditional branch on `d_ptr`.
Mingw's gcc rightly identified it as `Wmaybe-uninitialized`.

Fixes: QTBUG-147570
Pick-to: 6.12 6.11 6.8
Change-Id: I00177945159cd9b652c3e95a9d24775132b9789c
Reviewed-by: Artem Dyomin <artem.dyomin@qt.io>
Reviewed-by: Elias Toivola <elias.toivola@qt.io>