drm/i915: Use alphabetical names for transcoders too

Print the alphabetical name for transcoders. The code already used the
pipe_name() macro for transcoders, so I did the same. But we do have the
(unused) transcoder_name() macro which could be used instead.

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