drm/i915: rename intel_{en, dis}able_transcoder

To ironlake_{en,dis}able_pch_transcoder since these functions will be
different on Haswell/LPT and since the "transcoder" they {en,dis}able
is on the PCH.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
[danvet: again a small conflict because the fdi disable sequenc looks
a bit different here.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>