disp: msm: avoid double unmap during secure vid to secure UI transition

During secure video to secure ui transition, since
secure ui requires s2 direct translation, display driver
will detach the iommu device after unmap of the buffers
attached to the CB.

In this regard, upstream framework which can independently
request for buffer free to unmap on the buffer creates an
illegal state where in dma ops are attached and iommu device
is dettached.

Cache the dma ops before detaching the CB and set it to
NULL during iommu device detach, to avoid multiple unmap
calls on the buffer.

Change-Id: I11cd33f5f52b8c9c4f3a627c520bff57e340c8a4
Signed-off-by: Jayaprakash <jmadiset@codeaurora.org>