msm: display: maintain smmu for secure video

Currently video maps and unmaps the secure memory.
It could be too early to unmap it.
Rotator and MDP will maintain this, only unmap it when
no more needed. Memory managr has a refence count for it.

Change-Id: I5ca7cb09c0ad4b9e39808e937c2f50264727730c
Signed-off-by: Ken Zhang <kenz@codeaurora.org>