This is required to fix an issue on the Retina MBP where the eDP panel's
AUX channel isn't wired up to the HPD pin for the panel, causing our aux
code to bail out early.

From looking at various traces of the binary driver, it appears NVIDIA do
something very similar on at least all nv50+ chipsets during their
initialisation sequence.  So, hopefully this is safe.

Issue and fix initially tracked down by Ryan Bourgeois on fdo#51971.

Backported fix from reworked nouveau kernel module.
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>