drm/i915/guc/ct: Stop expecting multiple CT channels

The GuC supports having multiple CT buffer pairs and we designed our
implementation with that in mind. However, the different channels are not
processed in parallel within the GuC, so there is very little advantage
in having multiple channels (independent locks?), compared to the
drawbacks (one channel can starve the other if messages keep being
submitted to it). Given this, it is unlikely we'll ever add a second
channel and therefore we can simplify our code by removing the
flexibility.

v2: split substructure grouping to separate patch, improve docs (Michal)

Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: John Harrison <John.C.Harrison@Intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191217012316.13271-3-daniele.ceraolospurio@intel.com