drm/virtio: rework virtio_fence_signaled

drm/virtio: rework virtio_fence_signaled

virtio_gpu_fence_event_process sets the last_fence_id and
subsequently calls dma_fence_signal_locked(..).

dma_fence_signal_locked(..) sets DMA_FENCE_FLAG_SIGNALED_BIT,
which is actually checked before &dma_fence_ops.(*signaled) is
called.

The check for last_fence_id is therefore a bit redundant, and
it will not be sufficient to check the last_fence_id for multiple
synchronization timelines.  Remove it.

v3: add r-b tags

Signed-off-by: Gurchetan Singh <gurchetansingh@chromium.org>
Reviewed-by: Anthoine Bourgeois <anthoine.bourgeois@gmail.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20201201021623.619-2-gurchetansingh@chromium.org
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>