auxdisplay: panel: Replace deprecated strncpy() with strtomem_pad()

auxdisplay: panel: Replace deprecated strncpy() with strtomem_pad()

`strncpy` is deprecated and as such we should prefer more robust and
less ambiguous interfaces.

In this case, all of `press_str`, `repeat_str` and `release_str` are
explicitly marked as nonstring:
|   struct {	/* valid when type == INPUT_TYPE_KBD */
|           char press_str[sizeof(void *) + sizeof(int)] __nonstring;
|           char repeat_str[sizeof(void *) + sizeof(int)] __nonstring;
|           char release_str[sizeof(void *) + sizeof(int)] __nonstring;
|   } kbd;

... which makes `strtomem_pad` a suitable replacement as it is
functionally the same whilst being more obvious about its behavior.

Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings
Link: https://github.com/KSPP/linux/issues/90
Cc: linux-hardening@vger.kernel.org
Cc: Kees Cook <keescook@chromium.org>
Signed-off-by: Justin Stitt <justinstitt@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20230911-strncpy-drivers-auxdisplay-panel-c-v1-1-b60bd0ae8552@google.com
Signed-off-by: Kees Cook <keescook@chromium.org>