drm/i915: ELiminate intel_pipe_to_cpu_transcoder() from assert_fdi_tx()

drm/i915: ELiminate intel_pipe_to_cpu_transcoder() from assert_fdi_tx()

Let's start to eliminate intel_pipe_to_cpu_transcoder() so that
we can get rid of one more crtc->config usage (which we will want
to nuke as well).

In the case of assert_fdi_tx() we know that we're never
dealing with the EDP transcoder so we can simply replace
this with a cast.

v2: Fix poor English in comment

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191112163812.22075-3-ville.syrjala@linux.intel.com
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>