drm/i915: convert DDI_FUNC_CTL to transcoder

Because there's one instance of the register per CPU transcoder and
not per CPU pipe. This is another register that appeared for the first
time on Haswell, and even though its Haswell name is
PIPE_DDI_FUNC_CTL, it will be renamed to TRANS_DDI_FUNC_CTL, so let's
just use the new naming scheme before it confuses more people.

Notice that there's a big improvement on intel_ddi_get_hw_state due to
the new TRANSCODER_EDP.

V2: Also rename the register to TRANS_DDI_FUNC_CTL as suggested by
Damien Lespiau.

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>