Fix build with DRM and INVARIANTS enabled.

Fix build with DRM and INVARIANTS enabled.

The DRM drivers use the lockdep assertion macros with spinlock_t locks
which are backed by mutexes, not sx locks.  This causes compile
failures since you can't use sx_assert with a mutex.  Instead, change
the lockdep macros to use lock_class methods.  This works by assuming
that each LinuxKPI locking primitive embeds a FreeBSD lock as its
first structure and uses a cast to get to the underlying 'struct
lock_object'.

Reviewed by:	hselasky
MFC after:	2 weeks
Differential Revision:	https://reviews.freebsd.org/D20992