UPSTREAM: drm/virtio: add virtio_gpu_object_detach() function

The new function balances virtio_gpu_object_attach().

Also make virtio_gpu_cmd_resource_inval_backing() static and switch
call sites to the new virtio_gpu_object_attach() function.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20180829122026.27012-2-kraxel@redhat.com


(cherry picked from commit b3f13ec958a77497da76cc7a89d60b741b79ba22)
Signed-off-by: Greg Hartman <ghartman@google.com>
BUG: 139386237
Change-Id: I8e152b15e7361a14aae318b1710e03bafec60ab6