drm/i915/selftests: Fixup GuC FW negative test

Since:
0d4b78b3 ("drm/i915/guc: Assert we have the doorbell before setting it up")

We have asserts in GuC doorbell related functions, which is a good thing.
Unfortunately, we were using those to check whether GuC FW is refusing
to allocate invalid doorbell - which makes the test fail.
Well, it would make the test WARN, except we fumbled cleanup ordering
and eat the BUG_ON instead.
Let's keep the asserts and use the internal implementation in the test.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=107186Signed-off-by: Michał Winiarski <michal.winiarski@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Michel Thierry <michel.thierry@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180712112013.3253-1-chris@chris-wilson.co.ukSigned-off-by: Chris Wilson <chris@chris-wilson.co.uk>

drivers/gpu/drm/i915/selftests/intel_guc.c

@@ -196,19 +196,23 @@ static int igt_guc_clients(void *args)
 	}
 
 	unreserve_doorbell(guc->execbuf_client);
-	err = guc_clients_doorbell_init(guc);
+
+	__create_doorbell(guc->execbuf_client);
+	err = __guc_allocate_doorbell(guc, guc->execbuf_client->stage_id);
 	if (err != -EIO) {
 		pr_err("unexpected (err = %d)", err);
-		goto out;
+		goto out_db;
 	}
 
 	if (!available_dbs(guc, guc->execbuf_client->priority)) {
 		pr_err("doorbell not available when it should\n");
 		err = -EIO;
-		goto out;
+		goto out_db;
 	}
 
+out_db:
 	/* clean after test */
+	__destroy_doorbell(guc->execbuf_client);
 	err = reserve_doorbell(guc->execbuf_client);
 	if (err) {
 		pr_err("failed to reserve back the doorbell back\n");