Introduce tui_data_window::line_from_reg_element_no method

Introduce tui_data_window::line_from_reg_element_no method

This changes tui_line_from_reg_element_no to be a method on
tui_data_window, allowing for the removal of some uses of the
TUI_DATA_WIN global.  (Actually, in this particular patch, the number
of uses is not decreased, but rather the uses are moved to spots that
are already using the global, i.e., increasing the clustering.)

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

	* tui/tui-windata.c (tui_display_data_from)
	(tui_data_window::do_scroll_vertical): Update.
	* tui/tui-regs.h (tui_line_from_reg_element_no): Don't declare.
	* tui/tui-regs.c (tui_data_window::line_from_reg_element_no):
	Rename from tui_line_from_reg_element_no.
	(tui_display_registers_from_line): Update.
	* tui/tui-data.h (struct tui_data_window)
	<line_from_reg_element_no>: New method.