This change fixes below error:
Message 0: ID3D11DeviceContext::CSSetShaderResources: Resource being set
to CS shader resource slot 0 is still bound on output! Forcing to NULL.

Bug: angleproject:3658
Change-Id: I72d656cf61ffb7c39660c019eab980c39eafb70f
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1688307Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jiajia Qin <jiajia.qin@intel.com>