The current code uses in some instances enum transcoder for PCH
transcoders and enum pipe in others. This is error prone and clang
raises warnings like this:

drivers/gpu/drm/i915/intel_dp.c:3546:51: warning: implicit conversion
  from enumeration type 'enum pipe' to different enumeration type
  'enum transcoder' [-Wenum-conversion]
    intel_set_pch_fifo_underrun_reporting(dev_priv, PIPE_A, false);

Consistently use the type enum pipe for PCH transcoders.
Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20170717181403.57324-1-mka@chromium.org