drm/nouveau/nvif: refactor deprecated strncpy

drm/nouveau/nvif: refactor deprecated strncpy

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

A suitable replacement is `strscpy_pad` due to the fact that it
guarantees NUL-termination on the destination buffer whilst also
maintaining the NUL-padding behavior that `strncpy` provides. I am not
sure whether NUL-padding is strictly needed but I see in
`nvif_object_ctor()` args is memcpy'd elsewhere so I figured we'd keep
the same functionality.

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
Signed-off-by: Justin Stitt <justinstitt@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230914-strncpy-drivers-gpu-drm-nouveau-nvif-client-c-v1-1-dc3b3719fcb4@google.com