gdb: Don't call tui_enable too early.

gdb: Don't call tui_enable too early.

Calling tui_enable too early in tui_layout_command can leave the tui in
an enabled state if the user has entered an invalid layout name.
Instead postpone the call to tui_enable until later in
tui_set_layout_for_display_command just before the layout is changed.

gdb/ChangeLog:

	* tui/tui-layout.c (tui_layout_command): Move call to tui_enable
	into ...
	(tui_set_layout_for_display_command): ...here, before calling
	tui_set_layout.  Only set the layout if gdb has not already
	entered the TUI_FAILURE state.