drm/nv50/disp: prevent false output detection on the original nv50

Commit ea9197cc effectively enabled the
use of an improved DAC detection code, but introduced a regression on
the original nv50 chipset, causing a ghost monitor to be detected.

v2 (Ben Skeggs): the offending line was likely a thinko, removed it for
all chipsets (tested nv50 and nve6 to cover entire range) and added
some additional debugging.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=67382


Tested-by: Martin Peres <martin.peres@labri.fr>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Cc: <stable@vger.kernel.org> # 3.9+
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>