drm/nouveau/disp: fix dithering not being enabled on some eDP macbooks

commit a6a17859f1bdf607650ee055101f54c5f207762b upstream.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Cc: Maarten Lankhorst <maarten.lankhorst@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>