drm: Enforce BKL-less ioctls for modern drivers

With the last round of changes all ioctls called by modern drivers now
have their own locking. Everything else is only allowed for legacy
drivers and hence the lack of locking doesn't matter.

One exception is nouveau, due to the DRIVER_KMS_LEGACY_CONTEXT flag.
But that only works its magic on the context and bufs ioctls. And
drm_bufs.c is protected with dev->struct_mutex, and drm_context.c by
the same and dev->ctxlist_mutex. That should be all safe, and we can
finally mandata drm-bkl-less ioctls for everyone!

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161210215255.7765-3-daniel.vetter@ffwll.ch