Two simplifications in tui-layout.c

Two simplifications in tui-layout.c

This patch simplifies some code in tui-layout.c.

In show_layout, all the layout settings can be handled by a single
switch statement.  In show_source_disasm_command and
show_source_or_disasm_and_command, there is no need to check the
current layout, as the caller has already done so.

gdb/ChangeLog
2019-08-13  Tom Tromey  <tom@tromey.com>

	* tui/tui-layout.c (show_layout): Unify all layout cases into a
	single switch.
	(show_source_disasm_command, show_source_or_disasm_and_command):
	Don't check current layout.