drm/amdgpu: Use lockless gem BO free callback

No dev->struct_mutex anywhere to be seen.

Cc: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1461691808-12414-11-git-send-email-daniel.vetter@ffwll.ch