drm/i915: Use pipe_config's cpu_transcoder for reading encoder hw state

The get_config() functions for ddi and dp_mst, used to read the value
of cpu_transcoder from the crtc->config instead of the state passed as
an argument. On the hardware state readout path, that happens to work
since the proper value is written to it before encoder->get_config() is
called. However, in the check_crtc() path, the state will be read from
the cpu_transcoder in the software tracking, instead of the one just
read out from hw. Using the field in the supplied intel_crtc_state
should do the right thing in both cases.

v2: Fix intel_ddi_get_config() too. (Ander)

Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>