drm/msm/dp: clear bootup event notifier on power on

The bootup notifier is hooked up with the framework. Clear
the notifier once the DisplayPort has powered up. This avoids
framework to issue duplicate notification on cable connect or
disconnect.

Change-Id: Ifc424ee7048b8822eb4ffc5b91cf1e58013b7f6f
CRs-Fixed: 2133563
Signed-off-by: Ajay Singh Parmar <aparmar@codeaurora.org>