drm/virtio: implement prime export

Just run drm_prime_pages_to_sg() on the ttm pages list to get an
sg_table for export.  The pages list is created at object initialization
time, so there should be no need to handle an unpopulated page list.
Add a sanity check nevertheless.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Noralf Trønnes <noralf@tronnes.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20190227144441.6755-4-kraxel@redhat.com