Fix drawing custom colours for wxEnumProperty choice items in wxPG.

In order to draw choice items with custom colours there is necessary to merge custom cell object (wxPGCell) associated with individual item with default cell object (in wxPGProperty::GetDisplayInfo).
wxPGProperty::GetDisplayInfo function should return customized cell object instead of returning pointer to the default cell object only.

Closes #16509

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@77505 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775