drm/msm/dp: signal notification completion after power on

Signal the notification completion after the display power on
sequence has been executed. This will unblock the worker thread
that is waiting on this completion, and avoid unnecessarily
timing out even after a successful power on.

Change-Id: I9bb714891e946a2e60e1ded6bd462e51ba7f1817
Signed-off-by: Tatenda Chipeperekwa <tatendac@codeaurora.org>