Compiler Attributes: auxdisplay: panel: use __nonstring

Let gcc know these arrays are not meant to be NUL-terminated
by annotating them with the new __nonstring variable attribute;
and remove the comment since it conveys the same information.

Tested-by: Sedat Dilek <sedat.dilek@gmail.com> # on top of v4.19-rc5, clang 7
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>
Signed-off-by: Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>