The handling of virtqueue_add_sgs ENOSPC error is incorrect because
it can result in out-of-order virtqueue_add_sgs and break fences.
We never get ENOSPC anyway because the caller waits until there is
enough space (the other caller that did not wait was removed in
commit 32d6c2c5 ("drm/virtio: Rewrite virtio_gpu_queue_ctrl_buffer
using fenced version.")).  Remove the incorrect and unnecessary error
path.

This also adds a WARN_ON(ret) until we properly handle errors.
Signed-off-by: Chia-I Wu <olvaffe@gmail.com>
Cc: David Riley <davidriley@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20200205181955.202485-3-olvaffe@gmail.comSigned-off-by: Gerd Hoffmann <kraxel@redhat.com>