Introduce tui_data_window::last_regs_line_no method

Introduce tui_data_window::last_regs_line_no method

This changes tui_last_regs_line_no into a method on tui_data_window,
allowing the removal of uses of the TUI_DATA_WIN global.

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

	* tui/tui-regs.h (tui_last_regs_line_no): Don't declare.
	* tui/tui-regs.c (tui_data_window::last_regs_line_no): Rename from
	tui_last_regs_line_no.
	(tui_display_reg_element_at_line)
	(tui_display_registers_from_line): Update.
	* tui/tui-data.h (struct tui_data_window) <last_regs_line_no>: New
	method.