drm/virtio: cleanup queue functions

Make the queue functions return void, none of
the call sites checks the return value.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Chia-I Wu <olvaffe@gmail.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20190813082509.29324-2-kraxel@redhat.com