Change tui_addr_is_displayed into a method

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.