drm/virtio: move remaining virtio_gpu_notify calls

Move all remaining virtio_gpu_notify() calls from virtio_gpu_cmd_*
to the callers, for consistency reasons.

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/20200214125535.26349-7-kraxel@redhat.com