drm/i915: Print plane, pipe, port names as alphabetical insted of decimal

Alway use the alphabetical names in debug/error messages for planes,
pipes and ports, instead of using decimal numbers occasionally.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>