drm/virtio: use DRM_FB_HELPER_DEFAULT_OPS for fb_ops

This patch removes a TODO comment in the code. I do not know whether it
is still relevant.

Cc: David Airlie <airlied@linux.ie>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Stefan Christ <contact@stefanchrist.eu>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1479078208-25221-16-git-send-email-contact@stefanchrist.eu