drm/i915: Consistently use enum pipe for PCH transcoders

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