drm/virtio: Ensure a context has been created before sending the attach command

Currently, a context is ensured when new buffers are created, but not
when a buffer gets imported into this context.

Move the call to virtio_gpu_create_context() into
virtio_gpu_gem_object_open() so it applies to all these situations.

Signed-off-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>