drm/i915/guc: Move GuC log related functions into dedicated file

Functions supporting GuC logging capabilities were spread across
many files, with unnecessary exposures and mixed with unrelated
code. Dedicate file will make maintenance of all GuC functions
easier as more functions are coming to support GuC submissions.

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Arkadiusz Hiler <arkadiusz.hiler@intel.com>
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170113174157.104492-1-michal.wajdeczko@intel.com