Use enum pipe for PCH transcoders also in the FIFO underrun code.

Fixes the following new sparse warnings:
intel_fifo_underrun.c:340:49: warning: mixing different enum types
intel_fifo_underrun.c:340:49:     int enum pipe  versus
intel_fifo_underrun.c:340:49:     int enum transcoder
intel_fifo_underrun.c:344:49: warning: mixing different enum types
intel_fifo_underrun.c:344:49:     int enum pipe  versus
intel_fifo_underrun.c:344:49:     int enum transcoder
intel_fifo_underrun.c:397:57: warning: mixing different enum types
intel_fifo_underrun.c:397:57:     int enum pipe  versus
intel_fifo_underrun.c:397:57:     int enum transcoder
intel_fifo_underrun.c:398:17: warning: mixing different enum types
intel_fifo_underrun.c:398:17:     int enum pipe  versus
intel_fifo_underrun.c:398:17:     int enum transcoder

Cc: Matthias Kaehlcke <mka@chromium.org>
Fixes: a2196033 ("drm/i915: Consistently use enum pipe for PCH transcoders")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170901143123.7590-3-ville.syrjala@linux.intel.comReviewed-by: Chris Wilson <chris@chris-wilson.co.uk>