drm/i915/guc: Move GuC guc_id allocation under submission state sub-struct

Move guc_id allocation under submission state sub-struct as a future
patch will reuse the spin lock as a global submission state lock. Moving
this into sub-struct makes ownership of fields / lock clear.

v2:
 (Docs)
  - Add comment for submission_state sub-structure
v3:
 (John Harrison)
  - Fixup a few comments
v4:
 (John Harrison)
  - Fix typo

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/20211014172005.27155-2-matthew.brost@intel.com