msm: mdss: Fix race condition while iommu mapping from DSI flow

Avoid race condition which occurs when SMMU get detached from
secure flow and the iommu mapping happening from DSI flow.

Change-Id: Icaf68b8569c13f2a1617035b6c548270afd6a312
Signed-off-by: Althaf Neelanchirayil <aneelanc@codeaurora.org>
Signed-off-by: Nirmal Abraham <nabrah@codeaurora.org>