msm: mdss: fix possible race condition in mdp resource control

There might be a race condition where the early wake-up work item gets
preempted for the stop call when the resource state is already set to
off by the early wake-up. In this condition, the stop call clears the
ctx, leading to issues when the early wake-up work item runs again since
the ctx is not valid any more. Fix this condition by making sure that
stop call always waits for the early wake-up work item to finish.

Change-Id: I64aba71bb4c5602df9a524b77bd8bf3296dda012
Signed-off-by: Veera Sundaram Sankaran <veeras@codeaurora.org>