drm/virtio: rework virtio_gpu_object_create fencing

Rework fencing workflow.  Stop using ttm helpers, use the
virtio_gpu_array_* helpers instead.

Due to using the gem reservation object it is initialized and ready for
use before calling ttm_bo_init.  So we can simply use the standard
fencing workflow and drop the tricky logic which checks whenever the
command is in flight still.

v6: rewrite most of the patch.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Chia-I Wu <olvaffe@gmail.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20190829103301.3539-10-kraxel@redhat.com