drm/i915: convert CPU M/N timings to transcoder

Same thing as the previous commits. Not renaming this one since it
exists since way before Haswell.

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