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

drivers/gpu/drm/virtio/virtgpu_display.c

@@ -75,12 +75,9 @@ virtio_gpu_framebuffer_init(struct drm_device *dev,
 			    struct drm_gem_object *obj)
 {
 	int ret;
-	struct virtio_gpu_object *bo;
 
 	vgfb->base.obj[0] = obj;
 
-	bo = gem_to_virtio_gpu_obj(obj);
-
 	drm_helper_mode_fill_fb_struct(dev, &vgfb->base, mode_cmd);
 
 	ret = drm_framebuffer_init(dev, &vgfb->base, &virtio_gpu_fb_funcs);