drm/i915: Check for CPT and not !IBX in ironlake_disable_pch_transcoder()

ironlake_enaable_pch_transcoder() checks for CPT to see if it should
enable the timing override chicken bit, but
ironlake_disable_pch_transcoder() checks for !IBX to see if it should
clear the same bit. Change ironlake_disable_pch_transcoder() to check
for CPT as well to keep the two sides consistent.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1446146763-31821-8-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>