drm/i915/lspcon: Get DDC adapter via container_of() instead of cached ptr

We can use the container_of() magic to get to the DDC adapter, so no
need for caching a pointer to it. We'll also need to get at the intel_dp
ptr in the following patch, so add a helper that can be used for both
purposes.

Cc: Shashank Sharma <shashank.sharma@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1477326811-30431-8-git-send-email-imre.deak@intel.com