LinuxKPI: Make lockdep*_pin_lock macros useable for drm-kmod

Summary:

• Add dummy struct pin_cookie definition;
• Convert lockdep_pin_lock macro to function;
• Fix 'unused variable' compile-time errors;

MFC after: 1 week
Reviewers: hselasky, manu
Differential Revision: https://reviews.freebsd.org/D33295