drm/i915: use CPU and PCH transcoders on lpt_enable_pch_transcoder

... instead of using "pipe". As already explained in previous commits,
since Haswell/LPT cpu_transcoder, pch_transcoder and pipe are not the
same thing.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>