msm: mdss: prevent power off iommu if handoff is still pending

When continuous splash handoff is still pending, is not
needed to enable/release the iommu, since clocks and iommu
are already refcounted and enabled.
Add a check so by the time that aplash screen is still
on-going, we do not release the iommu.

Change-Id: I5ea948b2997aeac594f933ddca1096b755473884
Signed-off-by: Ingrid Gallardo <ingridg@codeaurora.org>