drm/i915/guc: Ensure multi-lrc fini breadcrumb math is correct

Realized that the GuC multi-lrc fini breadcrumb emit code is very
delicate as the math this code does relies on functions it calls to emit
a certain number of DWs. Add a few GEM_BUG_ONs to assert the math is
correct.

v2:
  - Rebase + resend for CI
 (Checkpatch)
  - Fix blank line warning

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/20220119210639.33053-1-matthew.brost@intel.com