This reverts commit f4c34b1e2a.
Simliar to commit a0cecc23cf Revert "drm/virtio: drop prime
import/export callbacks". We have to do the same with qxl,
for the same reasons (it breaks DRI3).
Drop the WARN_ON_ONCE().
Fixes: f4c34b1e2a ("drm/qxl: drop prime import/export callbacks")
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20190426053324.26443-1-kraxel@redhat.com
Acked-by: Daniel Vetter <daniel@ffwll.ch>