drm/armada: Use lockless gem BO free callback
No dev->struct_mutex anywhere to be seen.
Cc: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1464630800-30786-2-git-send-email-daniel.vetter@ffwll.ch