msm: mdss: synchronize recovery handler access

The recovery handler is used in the dsi event thread, and
the recovery handler variable was configured from a different
thread.  Earlier, there is no synchronization between the two
threads when accessing the recovery handler, causing kernel
panics.

Change-Id: Iee990276fdabd65e2e1f2e3c21ad574fc0d8a7bc
Signed-off-by: Xiaoming Zhou <zhoux@codeaurora.org>