drm/virtio: fix framebuffer sparse warning

virtio uses normal ram as backing storage for the framebuffer, so we
should assign the address to new screen_buffer (added by commit
17a7b0b4) instead of screen_base.
Reported-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

drivers/gpu/drm/virtio/virtgpu_fb.c

@@ -331,7 +331,7 @@ static int virtio_gpufb_create(struct drm_fb_helper *helper,
 	info->fbops = &virtio_gpufb_ops;
 	info->pixmap.flags = FB_PIXMAP_SYSTEM;
 
-	info->screen_base = obj->vmap;
+	info->screen_buffer = obj->vmap;
 	info->screen_size = obj->gem_base.size;
 	drm_fb_helper_fill_fix(info, fb->pitches[0], fb->depth);
 	drm_fb_helper_fill_var(info, &vfbdev->helper,