drm/sti: Use lockless gem BO free callback

With Benjanim's patch to remove the dev->struct_mutex cargo cult the
sti driver is now also entirely legacy locking free. Let's convert it
too.

Cc: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1464630800-30786-20-git-send-email-daniel.vetter@ffwll.ch