mdss: mdp: secure buffer is unmaped without iommu attached

For secure content/buffers, it is a requirement that IOMMU is attached
before un-mapping can be called, otherwise the scm call to unmap the
buffer will not be successful. In the current implementation it is
possible to unmap buffer while going into suspend without IOMMU being
attached which leads to secure playback failures in subsequent sessions.
Fix this by ensuring IOMMU is attached before buffer is freed/unmapped.

Change-Id: I29cdce834356b45f05fc75ec07d1c6fa0fcf2d69
Signed-off-by: Ujwal Patel <ujwalp@codeaurora.org>
Signed-off-by: Siva Yarravarapu <sivay@codeaurora.org>