drm/i915/crt: Populate connector->ddc

Populate connector->ddc, and thus create the "ddc" symlink
in sysfs for analog VGA connectors.

As a bonus we can replace a bunch of intel_gmbus_get_adapter()
lookups with just the connector->ddc pointer. Sadly one extra
lookup still remains due to the g4x DVI-I shenanigans. We could
perhaps consider borrowing the ddc proxy idea from SDVO to deal
with that in a perhaps nicer way, but can't really be bothered
right now at least. Also not sure exposing such a dual ddc bus
to userspace would be quite wise.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230829113920.13713-6-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>