drm/vc4: Use lockless gem BO free callback

No dev->struct_mutex anywhere to be seen.

Cc: Eric Anholt <eric@anholt.net>
Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1464630800-30786-16-git-send-email-daniel.vetter@ffwll.ch