drm/virtio: rework virtio_gpu_cmd_context_{attach, detach}_resource

Switch to the virtio_gpu_array_* helper workflow.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Chia-I Wu <olvaffe@gmail.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20190829103301.3539-13-kraxel@redhat.com