drm/i915/guc: Merge communication_stop and communication_disable

The only difference from the GuC POV between guc_communication_stop and
guc_communication_disable is that the former can be called after GuC
has been reset. Instead of having two separate paths, we can just skip
the call into GuC in the disabling path and re-use that.

Note that by using the disable() path instead of the stop() one there
are two additional changes in SW side for the stop path:

- interrupts are now disabled before disabling the CT, which is ok
  because we do not want interrupts with CT disabled;
- guc_get_mmio_msg() is called in the stop case as well, which is ok
  because if there are errors before the reset we do want to record
  them.

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/20191217012316.13271-1-daniele.ceraolospurio@intel.com