msm: mdss: fix the IO MMU page fault on 8x10

During display suspend, the ION memory got free before MDP
DMA stop.  Make sure MDP DMA shut down properly and free
the ION MMU after that.

Change-Id: Iac39a1dc5a9e63328283f0821b93ad3a214b3f6d
Signed-off-by: Xiaoming Zhou <zhoux@codeaurora.org>