drm/i915: Switch to using port stored in intel_encoder

Now that we have the port enum stored in intel_encoder, use that instead of
dereferencing intel_dig_port. Saves us a few locals.

struct intel_encoder variables have been renamed to be consistent and
convey type information.

v2:
Fix incorrect 'enum port' member names - s/attached_port/port

Signed-off-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Reviewed-by: Lyude <cpaul@redhat.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1474334681-22690-4-git-send-email-dhinakaran.pandiyan@intel.com