drm/i915: Fix up sdvo hpd pins for i965g/gm

Bspec seems to be full of lies, at least it disagress with reality:
Two systems corrobated that SDVO hpd bits are the same as on gen3.

v2: Update comment a bit.

Cc: Arthur Ranyan <arthur.j.runyan@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Tested-by: Chris Wilson <chris@chris-wilson.co.uk>
Reported-and-tested-by: Alex Fiestas <afiestas@kde.org>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=58405
Cc: stable@vger.kernel.org
Acked-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>