msm: wb: Ensure iommu reference is available in wb terminate

In static screen case on command mode, when wifi display is
disconnected, buffers don't get free as there is a probability
that iommu is not attached. Ensure that iommu is attached when
wifi display terminate is called.

Change-Id: I3cd92072cc44074b31766a5542a9b59e1a2bdad1
Signed-off-by: Jeevan Shriram <jshriram@codeaurora.org>