drm/i915: Simplify intel_ddi_get_encoder_port()

We no longer have any need to look up the intel_digital_port based
on the passed in intel_encoder, but we still want to look up the port.
Let's just move that logic into intel_ddi_get_encoder_port() and drop
the dig_port stuff.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1468328376-6380-9-git-send-email-ville.syrjala@linux.intel.com