Fix wxGetClientDisplayRect() when _NET_WORKAREA is not supported.

Don't intersect the total rectangle with the uninitialized work area one
resulting in an empty rectangle being returned from wxGetClientDisplayRect()
if X11 server doesn't support _NET_WORKAREA (this is the case for at least
Cygwin X11 server).

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@76433 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775