ext_external_objects: fix validation error on vertexbindingdescriptions
This fixes validation error VUID-vkCmdDraw-None-04007 thrown when running tests with validation layer.
Error is thrown because pipeline is setup with a binding description but vkCmdBindVertexBuffers is not called. Fix is to skip setting up binding description when vbo is not being used by the test.
Signed-off-by: Tapani Pälli tapani.palli@intel.com @hikiko