Move SalGenericData::GetSalDisplay() out of generic/gendata.hxx

...as it needs SalDisplay to be a complete type, but unx/saldisp.hxx depends on
X11 headers and can thus e.g. not be included in generic/gendata.hxx on Android.

Change-Id: Iec5f51408eef0d6eb7e2d04105a7408372b06079