drm/nouveau/nvkm: give each nvkm_event its own lockdep class

The vblank and nonstall events have some annoying interactions with DRM
locking, and aren't able to do certain things as a result.

However, other uses of event notifications don't have such requirements,
and upcoming patches take advantage of this for various improvements.

Having separate classes for each nvkm_event's spinlocks allows lockdep
to distinguish between them and avoid false-positives.

v2: __always_inline + comment

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>