drm/imx: Use lockless gem BO free callback

No dev->struct_mutex anywhere to be seen.

Cc: Sascha Hauer <s.hauer@pengutronix.de>
Cc: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Acked-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1461691808-12414-20-git-send-email-daniel.vetter@ffwll.ch