drm/i915: Don't offset the pin used for crt_ddc

Previously when converting the GMBUS pin to the GPIO reg, we would
offset the pin by one and then use the look-up table. Now that we first
try to use the GMBUS pin, we no longer need the offset and can use the
value from the VBIOS directly.

Reported-by: Carlos R. Mafra <crmafra2@gmail.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>