drm/virtio: Import prime buffers from other devices as guest blobs

drm/virtio: Import prime buffers from other devices as guest blobs

By importing scanout buffers from other devices, we should be able
to use the virtio-gpu driver in KMS only mode. Note that we attach
dynamically and register a move_notify() callback so that we can
let the VMM know of any location changes associated with the backing
store of the imported object by sending detach_backing cmd.

Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Cc: Rob Clark <robdclark@gmail.com>
Cc: Gurchetan Singh <gurchetansingh@chromium.org>
Cc: Chia-I Wu <olvaffe@gmail.com>
Signed-off-by: Vivek Kasireddy <vivek.kasireddy@intel.com>
Tested-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Reviewed-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
[dmitry.osipenko@collabora.com: added kref check to move_notify]
Link: https://patchwork.freedesktop.org/patch/msgid/20241126031643.3490496-5-vivek.kasireddy@intel.com