fix [#37059] OSX Debug - assert when using external screen with MacBookPro

This was old code, Blender now supports multiple screens.
The assert was breaking my builds with -DWITH_ASSERT_ABORT=ON

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk@60716 954f8c5b-7b00-dc11-b283-0030488c597c