drm/virtio: fix ring free check

If the virtio device supports indirect ring descriptors we need only one
ring entry for the whole command.  Take that into account when checking
whenever the virtqueue has enough free entries for our command.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Chia-I Wu <olvaffe@gmail.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20200207064653.14403-1-kraxel@redhat.com