drm/virtio: Remove set but not used variable 'bo'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/gpu/drm/virtio/virtgpu_display.c: In function 'virtio_gpu_framebuffer_init':
drivers/gpu/drm/virtio/virtgpu_display.c:78:28: warning:
 variable 'bo' set but not used [-Wunused-but-set-variable]
  struct virtio_gpu_object *bo;
                            ^

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1536285837-150460-1-git-send-email-yuehaibing@huawei.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>