Consolidate setting of current_layout

Consolidate setting of current_layout

Currently several functions in tui-layout.c set current_layout after
their work is done.  This moves this assignment to show_layout,
instead.

gdb/ChangeLog
2019-11-12  Tom Tromey  <tom@tromey.com>

	* tui/tui-layout.c (show_layout): Set current_layout.
	(show_source_disasm_command, show_data)
	(show_source_or_disasm_and_command): Don't set current_layout.

Change-Id: Id8b23797d68e607f0fcd6d29b8801869d40d1869