drm/msm/dsi-staging: synchronize DSI error recovery and power off

In the current implementation DSI error recovery sequence and
display power off threads can run independently, which might put
DSI driver and hardware in a bad state, causing unwanted failures
and stability issues. Fix the same by properly synchronizing both
the threads.

Change-Id: I7bacfdfb63a815d7850b0568a4dee5c84d1f6351
Signed-off-by: Tharun Raj Soma <tsoma@codeaurora.org>
Signed-off-by: Sandeep Panda <spanda@codeaurora.org>