QAxHostWidget: Remove dependency on qt_win_display_dc()

qt_win_display_dc() is a simple wrapper around GetDC(0) which is only
used here for redirected painting (grabbing) of a QAxHostWidget.
As the export might be changed/removed, expand the code.

Change-Id: Id00546282d37ae00baa36fdd22164e412d7e57f9
Reviewed-by: Andy Shaw <andy.shaw@qt.io>