msm: mdss: Protect iommu attach/detach with mutex for mdp3

In case of command mode panels we get iommu attach/detach call
from different threads. Hence protect the iommu attach/detach
operations with a mutex.

Change-Id: If18ecc18ffea3f8a88f48b91251523e8df29eda7
Signed-off-by: Shivaraj Shetty <shivaraj@codeaurora.org>