drm: move drm-lock API to drm_legacy.h

Same as the other legacy APIs, most of this is internal, so prefix it with
drm_legacy_* and move into drm_legacy.h.

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Reviewed-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>