msm: mdss: protect iommu refcount with mutex for mdp3

On mdp3 targets iommu refcount is not properly protected
by mutex lock. This is causing iommu to be detached while
hardware is fetching the data from the iommu mapped
address leading to SNOC errors. Add mutex lock while
using the iommu refcounts to fix the issue.

Change-Id: Ife59791b08b123f6b60d3ffa6d8061839244934c
Signed-off-by: Shivaraj Shetty <shivaraj@codeaurora.org>