drm/i915/guc: Improve clean up of busyness stats worker

The stats worker thread management was mis-matched between
enable/disable call sites. Fix those up. Also, abstract the
cancel/enable code into a helper function rather than replicating in
multiple places.

v2: Rename the helpers and wrap the enable as well as the cancel
(review feedback from Daniele).

Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Reviewed-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230217223308.3449737-2-John.C.Harrison@Intel.com