scripts/gdb: bail early if there are no generic PD

Avoid generating an exception if there are no generic power domain(s)
registered:

(gdb) lx-genpd-summary
domain                          status          children
    /device                                             runtime status
----------------------------------------------------------------------
Python Exception <class 'gdb.error'>: No symbol "gpd_list" in current context.
Error occurred in Python: No symbol "gpd_list" in current context.
(gdb) quit

[f.fainelli@gmail.com: correctly invoke gdb_eval_or_none]
  Link: https://lkml.kernel.org/r/20230327185746.3856407-1-f.fainelli@gmail.com
Link: https://lkml.kernel.org/r/20230323231659.3319941-1-f.fainelli@gmail.com


Fixes: 8207d4a8 ("scripts/gdb: add lx-genpd-summary command")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Cc: Jan Kiszka <jan.kiszka@siemens.com>
Cc: Kieran Bingham <kbingham@kernel.org>
Cc: Leonard Crestez <leonard.crestez@nxp.com>
Cc: Stephen Boyd <sboyd@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>