drm/i915/selftests: Declare we allocated the guc clients

Silence smatch over

drivers/gpu/drm/i915/selftests/intel_guc.c:135 igt_guc_init_doorbell_hw() error: we previously assumed 'guc->execbuf_client' could be null (see line 123)
drivers/gpu/drm/i915/selftests/intel_guc.c:142 igt_guc_init_doorbell_hw() error: we previously assumed 'guc->preempt_client' could be null (see line 123)

by asserting that we did succeed in creating the pair of clients for
testing.

References: 55bd6bd75717 ("drm/i915/selftests: Add a GuC doorbells selftest")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Michel Thierry <michel.thierry@intel.com>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171120211907.1649-1-chris@chris-wilson.co.uk
Reviewed-by: Michel Thierry <michel.thierry@intel.com>

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

@@ -131,6 +131,8 @@ static int igt_guc_init_doorbell_hw(void *args)
 		pr_err("Failed to create clients\n");
 		goto unlock;
 	}
+	GEM_BUG_ON(!guc->execbuf_client);
+	GEM_BUG_ON(!guc->preempt_client);
 
 	err = validate_client(guc->execbuf_client,
 			      GUC_CLIENT_PRIORITY_KMD_NORMAL, false);