drm/virtio: Fix warning in virtio_gpu_queue_fenced_ctrl_buffer.

Fix warning introduced with commit e1218b8c
("drm/virtio: Use vmalloc for command buffer allocations.")
from drm-misc-next.

Signed-off-by: David Riley <davidriley@chromium.org>
Reviewed-by: Chia-I Wu <olvaffe@gmail.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20190912160048.212495-1-davidriley@chromium.org


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