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.

Signed-off-by: Vignesh Raman <vignesh.raman@collabora.com>