drm/ci: enable lockdep detection

We have enabled PROVE_LOCKING (which enables LOCKDEP) in drm-ci.
This will output warnings when kernel locking errors are encountered
and will continue executing tests. To detect if lockdep has been
triggered, check the debug_locks value in /proc/lockdep_stats after
the tests have run. When debug_locks is 0, it indicates that lockdep
has detected issues and turned itself off. Check this value, and if
lockdep is detected, exit with an error and configure it as a warning
in GitLab CI.

GitLab CI ignores exit codes other than 1 by default. Pass the correct
exit code with variable FF_USE_NEW_BASH_EVAL_STRATEGY set to true or
exit on failure.

Also update the documentation.

Acked-by: Helen Koike <helen.fornazier@gmail.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250217053719.442644-4-vignesh.raman@collabora.com
Signed-off-by: Vignesh Raman <vignesh.raman@collabora.com>

Documentation/gpu/automated_testing.rst

@@ -115,6 +115,10 @@ created (eg. https://gitlab.freedesktop.org/janedoe/linux/-/pipelines)
 5. The various jobs will be run and when the pipeline is finished, all jobs
 should be green unless a regression has been found.
 
+6. Warnings in the pipeline indicate that lockdep
+(see Documentation/locking/lockdep-design.rst) issues have been detected
+during the tests.
+
 
 How to update test expectations
 ===============================