msm: mdss: Synchronize DSI off, BTA status and DSI event handler

Currently there will not be a race condition with ESD check and
dsi off sequences. But there can be a race condition with dsi_off
and DSI event handler as the mutex is removed from dsi_off. Add
the mutex back again in dsi_off and BTA status check.

Change-Id: I1ae6cccbcf255f5295d1fdb79de140d5615b45ad
Signed-off-by: Jayant Shekhar <jshekhar@codeaurora.org>