drm/virtio: batch resource creation

Move virtio_gpu_notify() to higher-level functions for
virtio_gpu_cmd_create_resource(), virtio_gpu_cmd_resource_create_3d()
and virtio_gpu_cmd_resource_attach_backing().

virtio_gpu_object_create() will batch commands and notify only once when
creating a resource.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Chia-I Wu <olvaffe@gmail.com>
Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20200214125535.26349-5-kraxel@redhat.com