drm/atmel: Use lockless gem BO free callback

No dev->struct_mutex anywhere to be seen.

Cc: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1464630800-30786-4-git-send-email-daniel.vetter@ffwll.ch