drm/msm/dsi: Don't use iommu for command TX buffer for DSIv2

We currently use iommu allocated DMA buffers for sending DSI commands.
DSIv2 doesn't have a port connected to the MDP iommu. Therefore, it
can't use iommu allocated buffers to fetch DSI commands.

Use a regular contiguous DMA buffer if we are DSIv2.

Signed-off-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>