drm/i915: base gmbus pin validity check on the gmbus pin map array

This will be helpful for adding future platforms. It is better to keep
the information in the single point of truth (the table) instead of
duplicating it into the validity function.

While at it, add dev_priv parameter to the function, also to prepare for
adding future platform support.

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