msm: mdss: add iommu attach in bus bandwidth control request

Add mdss_iommu_attach in mdss_bus_bandwidth_ctrl to
attach iommu on bus bandwidth enable request.
mdss_iommu_attach/mdss_iommu_dettach can happen in
separate thread, which may lead to a race condition
where attach/detach may happen at same time. Add lock
to properly synchronize the iommu attach/detach to
fix race condition.

Change-Id: I45c8ef1fa1fa1c2da9c6c3702bd007856e469b9d
Signed-off-by: Pawan Kumar <pavaku@codeaurora.org>