You will see one warning for each DCM not used in the clock_generator module. You can safely ignore these warnings because these DCMs are not being used and you do not have to set this parameter.
If you do not want to see the warning, you can set the C_DCMx_CLKIN_PERIOD parameter.
This problem has been fixed in the latest 10.1 Service Pack available at:
http://www.xilinx.com/support/download/index.htm The first service pack containing the fix is 10.1 Service Pack 2.