drm/nouveau/gr/gp107,gp108: implement workaround for HW hanging during init

Certain boards with GP107/GP108 chipsets hang (often, but randomly) for
unknown reasons during GR initialisation.

The first tell-tale symptom of this issue is:

nouveau 0000:01:00.0: bus: MMIO read of 00000000 FAULT at 409800 [ TIMEOUT ]

appearing in dmesg, likely followed by many other failures being logged.

Karol found this WAR for the issue a while back, but efforts to isolate
the root cause and proper fix have not yielded success so far.  I've
modified the original patch to include a few more details, limit it to
GP107/GP108 by default, and added a config option to override this choice.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Reviewed-by: Karol Herbst <kherbst@redhat.com>