drm/i915: convert PIPECONF to use transcoder instead of pipe

Because the PIPECONF register is actually part of the CPU transcoder,
not the CPU pipe.

Ideally we would also rename PIPECONF to TRANSCONF to remind people
that they should use the transcoder instead of the pipe, but let's
keep it like this for now since most Gens still name it PIPECONF.

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>