drm/i915: Remove distinction between DDI 2 vs 4 lanes

We never make use of the distinction between 2 vs 4 lanes so combine
them into a per port domain instead. This saves us a few bits in the
power domain mask. Change suggested by Ville.

Signed-off-by: Patrik Jakobsson <patrik.jakobsson@linux.intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1447084107-8521-7-git-send-email-patrik.jakobsson@linux.intel.com