drm/virtio: fix missing dma_fence_put() in virtio_gpu_execbuffer_ioctl()

We should put the reference count of the fence after calling
virtio_gpu_cmd_submit(). So add the missing dma_fence_put().

Fixes: 2cd7b6f0

 ("drm/virtio: add in/out fence support for explicit synchronization")
Co-developed-by: Xin He <hexin.op@bytedance.com>
Signed-off-by: Xin He <hexin.op@bytedance.com>
Signed-off-by: Qi Liu <liuqi.16@bytedance.com>
Reviewed-by: Muchun Song <songmuchun@bytedance.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20200721101647.42653-1-hexin.op@bytedance.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>