drm/i915/guc: Return NULL for missing multi-lrc parent

multi_lrc_create_parent() returns ERR_PTR(0) when there are not enough
engines in the class to create a parallel context.  ERR_PTR(0) evaluates
to NULL, and the only caller already treats NULL as the non-error
"not enough engines" case.

Return NULL directly to make the non-error path explicit.

Signed-off-by: Linmao Li <lilinmao@kylinos.cn>
Signed-off-by: Tvrtko Ursulin <tursulin@ursulin.net>
Link: https://lore.kernel.org/r/20260706071412.559909-1-lilinmao@kylinos.cn

drivers/gpu/drm/i915/gt/uc/selftest_guc_multi_lrc.c

@@ -44,7 +44,7 @@ multi_lrc_create_parent(struct intel_gt *gt, u8 class,
 	}
 
 	if (i <= 1)
-		return ERR_PTR(0);
+		return NULL;
 
 	logical_sort(siblings, i);