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>