msm: mdss: handle synchronization issues during DSI debugfs read/write

Handle race condition during read/write operations to DSI debugfs nodes
related to DSI panel ON/OFF commands.

Change-Id: I29c4ad74bf21d4cb5362565e902a682fe7263147
Signed-off-by: Padmanabhan Komanduru <pkomandu@codeaurora.org>