drm/i915/guc: Move fields protected by guc->contexts_lock into sub structure

To make ownership of locking clear move fields (guc_id, guc_id_ref,
guc_id_link) to sub structure guc_id in intel_context.

Reviewed-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210909164744.31249-22-matthew.brost@intel.com