msm: mdss: fix race condition in mdp debugfs

Fix race condition in mdp debugfs properties
during the read and write of the panel and
mdp registers. This race condition can cause
accessing memory out bounderies.

Change-Id: I97a90a154237343d4aaf237c11f525bcc2c3a8e3
Signed-off-by: Ingrid Gallardo <ingridg@codeaurora.org>