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

[[e-mail]: correctly invoke gdb_eval_or_none]
Link: https://lkml.kernel.org/r/[e-mail]
Link: https://lkml.kernel.org/r/[e-mail]
Fixes: 8207d4a88e1e ("scripts/gdb: add lx-genpd-summary command")
Signed-off-by: Florian Fainelli <[e-mail]>
Cc: Jan Kiszka <[e-mail]>
Cc: Kieran Bingham <[e-mail]>
Cc: Leonard Crestez <[e-mail]>
Cc: Stephen Boyd <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

scripts/gdb/linux/genpd.py

···
 import gdb
 import sys
 
-from linux.utils import CachedType
+from linux.utils import CachedType, gdb_eval_or_none
 from linux.lists import list_for_each_entry
 
 generic_pm_domain_type = CachedType('struct generic_pm_domain')
···
             gdb.write('    %-50s  %s\n' % (kobj_path, rtpm_status_str(dev)))
 
     def invoke(self, arg, from_tty):
+        if gdb_eval_or_none("&gpd_list") is None:
+            raise gdb.GdbError("No power domain(s) registered")
         gdb.write('domain                          status          children\n');
         gdb.write('    /device                                             runtime status\n');
         gdb.write('----------------------------------------------------------------------\n');