msm: mdss: fix to cancel pending early wakeup work during stop

Fix to flush any pending early wakeup work item and
prevent to schedule more if we are in the middle of
an stop request.

Change-Id: Id87eb8d37f68584811da5ce9d9dd87f35529689e
Signed-off-by: Ingrid Gallardo <ingridg@codeaurora.org>