drm/msm/dp: fix hot plug event notification

Fill the uevent data correctly so that the listening
user modules can parse the data correctly and handle
the hot plug events from sink.

CRs-Fixed: 2072336
Change-Id: If7b5aa3a0bee058652e7ed93e2740e17b2ce6940
Signed-off-by: Ajay Singh Parmar <aparmar@codeaurora.org>