drm/msm/dsi-staging: update ESD thread disable sequence

Currently to synchronize ESD check and DSI disable, ESD
thread acquires display mutex before the check starts.
But this is causing performance issues when atomic check
also tries to acquire the display mutex and blocks on ESD
thread if the later has already acquired it.
Fix this race condition by removing display mutex lock from
ESD thread and instead synchronize ESD check and DSI disable
by always ensuring that ESD check has been stopped before
DSI disable sequence kicks in.

Change-Id: I5a0b44a7cbf01648303eb85c7a08a6a62c1ef94e
Signed-off-by: Sandeep Panda <spanda@codeaurora.org>