msm: mdss: protect iommu attached status variable with iommu_lock

Commit dd7ad27e ("msm: mdss: fix
race condition between iommu attach and sending DCS cmds") protected
the iommu attach/detach to fix possible race condition between the
display thread and DCS cmd sending from ESD or some other thread with
iommu_lock. But this still leaves possibility of race condition based
on iommu_attached status flag.
  -> Display thread finishes the attach and releases the iommu lock
  -> DCS cmd thread gets the iommu lock and checks iommu_attached
     status before display thread could change it.
Protect the iommu_attached status variable along with iommu
attach/detach to avoid such cases.

Change-Id: I42d0ced5cbfd741cf02ad71c80c4f01dde37f647
Signed-off-by: Veera Sundaram Sankaran <veeras@codeaurora.org>