Introduce reset_locator function in tui-layout.c

Introduce reset_locator function in tui-layout.c

init_and_make_win in tui-layout.c is now only called for the locator
-- earlier changes have made most of the cases here obsolete.  This
patch removes init_and_make_win and introduces a reset_locator
function.  Window creation is now much simpler to follow, because it
is no longer quite so dynamic.  (Though it will become even simpler in
coming patches.)

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

	* tui/tui-layout.c (show_source_disasm_command): Use
	reset_locator.
	(reset_locator): New function.
	(init_and_make_win): Remove.
	(show_source_or_disasm_and_command): Use reset_locator.