[gdb/tui] Show regs when switching to regs layout

[gdb/tui] Show regs when switching to regs layout

When starting gdb in CLI mode, running to main and switching into the TUI regs
layout:
...
$ gdb -q a.out -ex start -ex "layout regs"
...
we get:
...
+---------------------------------+
|                                 |
| [ Register Values Unavailable ] |
|                                 |
+---------------------------------+
...

Fix this by handling this case in tui_data_window::rerender.

Tested on x86_64-linux.

Approved-By: Tom Tromey <tom@tromey.com>
PR tui/28600
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=28600