drm/i915: index gmbus tables using the pin pair number

Index the gmbus tables directly using the pin instead of having a
confusing "port = i + 1" mapping. This finishes off removing the "gmbus
port" as a notion, and leaves us with just the "gmbus pin".

As pin 0 is invalid by definition and the gmbus tables will have a gap
at that index, add pin validity check to all the loops. This will be
benefitial for supporting platforms that have different numbers of pins,
or gaps.

v2: s/GMBUS_PIN_MAX/GMBUS_NUM_PINS/ (Ville, Daniel)

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>