Introduce two more tui_data_window methods

Introduce two more tui_data_window methods

This changes tui_display_registers_from and
tui_display_registers_from_line to be methods on tui_data_window,
allowing for the removal of more uses of the TUI_DATA_WIN global.

gdb/ChangeLog
2019-07-17  Tom Tromey  <tom@tromey.com>

	* tui/tui-regs.h (tui_display_registers_from)
	(tui_display_registers_from_line): Don't declare.
	* tui/tui-windata.c (tui_data_window::display_all_data)
	(tui_data_window::refresh_all)
	(tui_data_window::do_scroll_vertical): Update.
	* tui/tui-regs.c (tui_data_window::display_registers_from): Rename
	from tui_display_registers_from.
	(tui_display_reg_element_at_line): Update.
	(tui_data_window::display_registers_from_line): Rename from
	tui_display_registers_from_line.
	* tui/tui-data.h (struct tui_data_window) <display_registers_from,
	display_registers_from_line>: New methods.