drm/virtio: set up virtqueue sgs before locking

sgs setup does not need to be in the critical section.

Signed-off-by: Chia-I Wu <olvaffe@gmail.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20200205181955.202485-7-olvaffe@gmail.com


Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>