drm/virtio: pass gem reservation object to ttm init

With this gem and ttm will use the same reservation object,
so mixing and matching ttm / gem reservation helpers should
work fine.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Chia-I Wu <olvaffe@gmail.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20190829103301.3539-2-kraxel@redhat.com