drm/i915: Rename intel_ddi_enable_pipe_func() to transcoder_func()

We are really talking about the transcoder function here and the disable
version uses trancoder in its name already, so let's try to be
consistent.

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