fbdev: msm: wait for frame complete before triggering dma cmd

In dual dsi case while updating backlight through dcs commands
there is a chance of getting both the readptr and dma_tx interrupt
very close, due to which DSI0 may start cmd mdp transfer and
DSI1 will start cmd dma transfer and cause deadlock.
To fix the issue do not trigger dma commands close to TE.

Change-Id: I38536477a1ab71167a5e315a2b2c4a2b0c215b4a
Signed-off-by: Raghavendra Ambadas <rambad@codeaurora.org>