tools/delaytop: improve error handling for missing PSI support

Enhanced display logic to conditionally show PSI information only when
successfully read, with helpful guidance for users to enable PSI support
(psi=1 cmdline parameter).

Link: https://lkml.kernel.org/r/20250907001417537vSx6nUsb3ILqI0iQ-WnGp@zte.com.cn


Signed-off-by: Fan Yu <fan.yu9@zte.com.cn>
Reviewed-by: xu xin <xu.xin16@zte.com.cn>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>