drm/i915: split ironlake_write_infoframe into ibx_ and cpt_

The registers are on the PCH, so use the PCH name instead of the CPU
name. Also, the way this function is implemented is really only for
CPT and PPT. For now, both functions have the same implementations:
the next patch will fix ibx_write_infoframe.

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