drm/msm/dsi: set cmd dma done mask before triggering cmds

Make sure that cmd dma done mask is set before sending
dma commands. This will make sure that we don't timeout
if the refcnt's are not properly handled. Many oem's
have their own customizations around this which maynot
handle the refcnt's correctly.

Change-Id: If7f5ed1fae20b57f6e9147cae2caa3c5097466c9
Signed-off-by: Vara Reddy <varar@codeaurora.org>