drm/virtio: implement context init: support init ioctl

drm/virtio: implement context init: support init ioctl

This implements the context initialization ioctl.  A list of params
is passed in by userspace, and kernel driver validates them.  The
only currently supported param is VIRTGPU_CONTEXT_PARAM_CAPSET_ID.

If the context has already been initialized, -EEXIST is returned.
This happens after Linux userspace does dumb_create + followed by
opening the Mesa virgl driver with the same virtgpu instance.

However, for most applications, 3D contexts will be explicitly
initialized when the feature is available.

Signed-off-by: Anthoine Bourgeois <anthoine.bourgeois@gmail.com>
Acked-by: Lingfeng Yang <lfy@google.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20210921232024.817-6-gurchetansingh@chromium.org
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>