drm/etnaviv: Use lockless gem BO free callback

No dev->struct_mutex anywhere to be seen.

Cc: Christian Gmeiner <christian.gmeiner@gmail.com>
Cc: Russell King <rmk+kernel@arm.linux.org.uk>
Cc: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Acked-by: Lucas Stach <l.stach@pengutronix.de>
Acked-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1461691808-12414-17-git-send-email-daniel.vetter@ffwll.ch