drm/i915/guc: Use correct context lock when callig clr_context_registered

s/ce/cn/ when grabbing guc_state.lock before calling
clr_context_registered.

Fixes: 0f797650

 ("drm/i915/guc: Rework and simplify locking")
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211214170500.28569-2-matthew.brost@intel.com