drm/tegra: Use lockless gem BO free callback

No dev->struct_mutex anywhere to be seen.

Cc: Thierry Reding <thierry.reding@gmail.com>
Cc: Terje Bergström <tbergstrom@nvidia.com>
Cc: linux-tegra@vger.kernel.org
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Acked-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1461691808-12414-28-git-send-email-daniel.vetter@ffwll.ch