drm/i915: Clean up the CPT DP .get_hw_state() port readout

Define a TRANS_DP_PIPE_TO_PORT() to make the CPT DP .get_hw_state()
pipe readout neater.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>