msm: mdss: add minimum bus vote during IOMMU attach

On 8939, 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: I30a70ca0b9d32a5ada449e620589378b3aea6ac3
Signed-off-by: Padmanabhan Komanduru <pkomandu@codeaurora.org>
Signed-off-by: Jayant Shekhar <jshekhar@codeaurora.org>