drm/i915: Use enum pipe for PCH transcoders

One more place where we've failed to switch to enum pipe when talking
about PCH transcoders.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171010125556.25086-1-ville.syrjala@linux.intel.com


Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>