drm/i915/guc: Extend deregistration fence to schedule disable

Extend the deregistration context fence to fence whne a GuC context has
scheduling disable pending.

v2:
 (John H)
  - Update comment why we check the pin count within spin lock

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/20210721215101.139794-11-matthew.brost@intel.com