drm/i915: Fix intel_pipe_to_cpu_transcoder for UMS

We don't have all the drm_crtc&co hanging around in that case.

This regression has been introduced in

commit 391f75e2
Author: Ville Syrjälä <ville.syrjala@linux.intel.com>
Date:   Wed Sep 25 19:55:26 2013 +0300

    drm/i915: Fix pre-CTG vblank counter

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=69521


Cc: stable@vger.kernel.org (for 3.13 only)
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>