Don't draw register sub windows outside the visible area

Don't draw register sub windows outside the visible area

If the regs window is not big enough to show all registers, the
registers whose values changed are always drawn, even if they are not
in the currently visible area.

So this marks the invisible register sub windows with y=0, and skips
their rerender call in check_register_values.

gdb/ChangeLog:

2021-02-07  Hannes Domani  <ssbssa@yahoo.de>

	* tui/tui-regs.c (tui_data_window::display_registers_from):
	Mark invisible register sub windows.
	(tui_data_window::check_register_values): Ignore invisible
	register sub windows.