drm/i915/guc: Use system workqueue for log capture

We only employ a single task for log capture, and created a workqueue
for the purpose of ensuring we had a high priority queue for low
latency. We can simply use the system_highpri_wq and avoid the
complication with creating our own admist the maze of mutexes.
(Currently we create the wq early before we even know we need it in
order to avoid trying to create it on demand while we hold the logging
mutex.)

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Michał Winiarski <michal.winiarski@intel.com>
Reviewed-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Reviewed-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190713100016.8026-1-chris@chris-wilson.co.uk