Change tui_addr_is_displayed into a method
    
    This changes tui_addr_is_displayed to be a method on
    tui_disasm_window, now that it is obvious that it can only be called
    for this type.
    
    gdb/ChangeLog
    2019-08-15  Tom Tromey  <tom@tromey.com>
    
    	* tui/tui-winsource.h (tui_addr_is_displayed): Don't declare.
    	* tui/tui-winsource.c (tui_addr_is_displayed): Move to
    	tui-disasm.c.
    	* tui/tui-disasm.h (struct tui_disasm_window) <addr_is_displayed>:
    	Declare.
    	* tui/tui-disasm.c (tui_disasm_window::addr_is_displayed): New
    	method.
    	(tui_disasm_window::maybe_update): Update.