msm: mdss: Fix potential race condition in assertive display

Assertive display can be turned off between releasing ad->lock and
aquiring mfd->lock. Store the value of mfd, while under ad->lock to prevent
null dereference.

Change-Id: I261d112457a7a969e5b2c2f6a8b89b08b6235d3b
Signed-off-by: Carl Vanderlip <carlv@codeaurora.org>