drm/i915/lspcon: Add dp_to_lspcon helper()

We need to get to LSPCON in the next patch, so factor out the helper for
it. While at it also remove the redundant GEN9 check.

Cc: Shashank Sharma <shashank.sharma@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Shashank Sharma <shashank.sharma@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1479755707-29596-3-git-send-email-imre.deak@intel.com