Fix latent display bug in tui_data_window

Fix latent display bug in tui_data_window

tui_data_window creates new curses windows, but does not pass in
coordinates relative to the data window's origin.  This means that the
data window could only ever be displayed as the topmost window in a
layout.  This is not a currently problem, because all the existing
layouts do this; but a subsequent patch will add user-defined layouts,
which could do otherwise.

gdb/ChangeLog
2020-02-22  Tom Tromey  <tom@tromey.com>

	* tui/tui-regs.c (tui_data_window::display_registers_from): Use
	correct coordinates.

Change-Id: I5101f2b2869557b87381ebdeebd9b7fd28687831