drm/i915/guc: Only rely on own CTB size

In upcoming GuC firmware, CTB size will be removed from the CTB
descriptor so we must keep it locally for any calculations.

While around, improve some debug messages and helpers.

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210603051630.2635-10-matthew.brost@intel.com