msm: mdss: Cleanup iommu ref count implementation

With current iommu ref count implementation, iommu is
attached at points when only needed and detached once
finished. Iommu reference count in overlay pan display
path is currently missing hence add it. Also remove
code repetitions and unnecessary error checks while
decreasing iommu ref counts in overlay and writeback
path.

Change-Id: If1aed768e5d704dd6c9d4bfd17c1ec368c8d8711
Signed-off-by: Jayant Shekhar <jshekhar@codeaurora.org>