Introduce tui_data_window::first_reg_element_no_inline

Introduce tui_data_window::first_reg_element_no_inline

This changes tui_first_reg_element_no_inline to be a method on
tui_data_window.  This again moves uses of the TUI_DATA_WIN global
from this function into other functions that are already using the
global.

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

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