drm/i915: Don't use intel_pipe_to_cpu_transcoder() when there's a pipe config around

No point in doing the crtc->pipe->crtc->config->cpu_transcoder dance
when we can just do crtc->config->cpu_transcoder.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1446146763-31821-2-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>