msm: mdss: wait for dma done before issuing dsi bta request

In the command mode panel, the mdp pixel data is using the
dsi command mode engine which is shared with the dsi dma
transaction.  This change makes sure there is no pending
mdp dma before issuing the dsi bta request.

Change-Id: Ida99e973c2bf73757726134ba7e2ad2df9e6eb02
Signed-off-by: Xiaoming Zhou <zhoux@codeaurora.org>