drm/i915/selftests: Fixup a missing legacy_idx

Grr, missed one*. For using the legacy engine map, we should use
engine->legacy_idx. Ideally, we should know the intel_context in the
selftest and avoid all the fiddling around with unwanted GEM contexts.

* In my defence, the conflict was added in another patch after it was
tested by CI.

v2: mock engines needs legacy love as well

Fixes: f1c4d157ab ("drm/i915: Fix up the inverse mapping for default ctx->engines[]")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190808194525.9410-2-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/selftests/i915_request.c

@@ -339,7 +339,7 @@ static int __igt_breadcrumbs_smoketest(void *arg)
 
 			mutex_lock(BKL);
 
-			ce = i915_gem_context_get_engine(ctx, t->engine->id);
+			ce = i915_gem_context_get_engine(ctx, t->engine->legacy_idx);
 			GEM_BUG_ON(IS_ERR(ce));
 			rq = t->request_alloc(ce);
 			intel_context_put(ce);

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

@@ -213,6 +213,7 @@ struct drm_i915_private *mock_gem_device(void)
 	if (mock_engine_init(i915->engine[RCS0]))
 		goto err_context;
 
+	intel_engines_driver_register(i915);
 	mutex_unlock(&i915->drm.struct_mutex);
 
 	WARN_ON(i915_gemfs_init(i915));