drm/i915: put the right cpu_transcoder into pipe_config for hw state readout

This hack is getting a bit messy, but this plugs the leak for now
until we have the cpu_transcoder properly pipe_config'ed.

Cc: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>