drm: Mark authmagic ioctls as unlocked

All protected by dev->master_mutex. And there's no driver callbacks,
which means no need to sync with old dri1 horror show drivers at all.
Hence safe to drop the drm legacy BKL from these paths.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1466148814-8194-5-git-send-email-daniel.vetter@ffwll.ch