drm/i915/guc: Ensure GuC schedule operations do not operate on child contexts

In GuC parent-child contexts the parent context controls the scheduling,
ensure only the parent does the scheduling operations.

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/20211014172005.27155-10-matthew.brost@intel.com