This reverts commit f4c34b1e.

Simliar to commit a0cecc23 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: f4c34b1e ("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.comAcked-by: Daniel Vetter <daniel@ffwll.ch>