Introduce tui_data_window::display_reg_element_at_line method

Introduce tui_data_window::display_reg_element_at_line method

This changes tui_display_reg_element_at_line to be a method on
tui_data_window, allowing for the removal of some uses of the
TUI_DATA_WIN global.

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

	* tui/tui-regs.c (tui_data_window::display_reg_element_at_line):
	Rename from tui_display_reg_element_at_line.
	(tui_data_window::display_registers_from_line): Update.
	* tui/tui-data.h (struct tui_data_window)
	<display_reg_element_at_line>: New method.