mesa: pass correct constant type to _mesa_fetch_state()

Fixes assorted warnings about float vs. gl_constant_value pointers.

src/mesa/program/prog_statevars.c

       if (paramList->Parameters[i].Type == PROGRAM_STATE_VAR) {
          _mesa_fetch_state(ctx,
 			   paramList->Parameters[i].StateIndexes,
-                           paramList->ParameterValues[i]);
+                           &paramList->ParameterValues[i][0].f);
       }
    }
 }