drm/nouveau: fix POST detection for certain chipsets

We totally fail at detecting un-POSTed chipsets prior to G80.  This commit
changes the pre-G80 POST detection to read the programmed horizontal total
from CRTC 0, and assume the card isn't POSTed if it's 0.

NVIDIA use some other heuristics more similar to what we do on G80, but I
wasted quite a long time trying to figure out the exact specifics of what
they do so we can try this for a bit instead.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>