msm: mdss: fix concurrency between add vsync handler and stop

If command mode interface is being stopped, adding vsync handler can
cause the clocks to go on and cause improper state after.
There are checks for panel off inside add_vsync_handler however, there
needs to be synchronization while the stop is in progress.

Change-Id: I99bfca53a18bc818eecc94007357abdf4a251e50
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>