single tenant IMC missing RBAC for the leases

[yuzisun]

Describe the bug

E0205 23:44:58.029305       1 leaderelection.go:332] error retrieving resource lock knative-eventing/inmemorychannel-dispatcher.knative.dev-eventing-pkg-reconciler-inmemorychannel-dispatcher.reconciler.00-of-01: leases.coordination.k8s.io "inmemorychannel-dispatcher.knative.dev-eventing-pkg-reconciler-inmemorychannel-dispatcher.reconciler.00-of-01" is forbidden: User "system:serviceaccount:default:imc-dispatcher" cannot get resource "leases" in API group "coordination.k8s.io" in the namespace "knative-eventing"

Expected behavior
should have the proper RBAC for HA when running imc dispatcher in tenant namespace

To Reproduce
Create imc channel with eventing.knative.dev/scope: namespace and then you will see the error in imc dispatcher log in tenant namespace

apiVersion: messaging.knative.dev/v1
kind: InMemoryChannel
metadata:
  annotations:
    eventing.knative.dev/scope: namespace
  name: kf-test

Knative release version
0.17.0

Additional context
Add any other context about the problem here such as proposed priority

[yuzisun]

/cc @lionelvillard

[yuzisun]

@lberk Any update on the issue ? is this going to be fixed in 0.21.0?