msm: mdss: IOMMU ref count implementation

Currently MDP is attaching/detaching iommu based on
bus bandwidth requests. This can lead to performance
issues, so implement ref count on iommu such that
iommu is attached where needed.

Change-Id: Ic4d35e2dc7f83291d1ab93d3e0109a2d69c98844
Signed-off-by: Jayant Shekhar <jshekhar@codeaurora.org>
Signed-off-by: Vineet Bajaj <vbajaj@codeaurora.org>
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>
Signed-off-by: Vineet Bajaj <vbajaj@codeaurora.org>