msm: mdss: add minimum bus vote during IOMMU attach

On 8909, we need to have SYS_MM_NOC clock enabled to be able
to perform IOMMU map/unmap transactions. Hence, add a minimum
bandwidth vote during iommu attach and remove the vote during
iommu detach.

Change-Id: I25a8fc826bf532917c0091a324ba7e7803e3c601
Signed-off-by: Sandeep Panda <spanda@codeaurora.org>