drm/i915/guc: Don't send policy update for child contexts.

The GuC FW applies the parent context policy to all the children,
so individual updates to the children are not supported and we
should not send them.

Note that sending the message did not have any functional consequences,
because the GuC just drops it and logs an error; since we were trying
to set the child policy to match the parent anyway the message being
dropped was not a problem.

Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: John Harrison <john.c.harrison@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/20220728003339.2361010-1-daniele.ceraolospurio@intel.com