The link_design command should be used only when reading an edif netlist.link_design after you do a read_edif, and should primarily be used for linking the edif netlist. You do not need to use a link_design command when using synth_design.
The synth_design will take care of the addition of the top module, part number and also the linking of the design files with reference to the top. The design files include pure RTL files and also mixed design (RTL/Netlist) files. In mixed design (RTL & Netlist), synth_design automatically links the netlist, and hence link_design is not needed.
Please refer topage 49 of UG892 (Vivado Design Flow Overview)for more information.