Display "main" on initial TUI startup

I noticed that even when there's a symbol file, "tui enable" won't
show "main" by default.  I think it should, and this patch fixes this.

gdb/ChangeLog
2019-12-20  Tom Tromey  <tom@tromey.com>

	* tui/tui.c (tui_enable): Call tui_display_main.

gdb/testsuite/ChangeLog
2019-12-20  Tom Tromey  <tom@tromey.com>

	* gdb.tui/list.exp: Check for source on initial listing.

Change-Id: Ic7bfc930e1179f5b61111e30a2dae46a98b00064