Turn tui_first_data_item_displayed into a method

Turn tui_first_data_item_displayed into a method

tui_first_data_item_displayed is only called from tui_data_window
methods, so turn it into a method as well.

gdb/ChangeLog
2019-06-25  Tom Tromey  <tom@tromey.com>

	* tui/tui-windata.h (tui_first_data_item_displayed): Don't
	declare.
	* tui/tui-windata.c (tui_data_window::first_data_item_displayed):
	Rename from tui_first_data_item_displayed.  Update.
	(tui_data_window::refresh_all)
	(tui_data_window::do_scroll_vertical): Update.
	* tui/tui-data.h (struct tui_data_window)
	<first_data_item_displayed>: Declare new method.