msm: mdss: fix autorefresh disable sequence

Sequence to disable autorefresh when handoff is
done to the kernel is missing to wait for the
previous transaction to finish. This change
makes sure any previous on-going transaction
finishes before disabling autorefresh.

Change-Id: I547aea5cc82989d03963b2153e2561092fd7b36c
Signed-off-by: Ingrid Gallardo <ingridg@codeaurora.org>