drm/i915/guc: Insert fence on context when deregistering

Sometimes during context pinning a context with the same guc_id is
registered with the GuC. In this a case deregister must be done before
the context can be registered. A fence is inserted on all requests while
the deregister is in flight. Once the G2H is received indicating the
deregistration is complete the context is registered and the fence is
released.

v2:
 (John H)
  - Fix commit message

Cc: John Harrison <john.c.harrison@intel.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: John Harrison <john.c.harrison@intel.com>
Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210721215101.139794-8-matthew.brost@intel.com