msm: mdss: dp: handle fast attention events

Sink may issue attention event faster then it can be processed by
source. Create a list of attention events and process them one by
one in the order on workqueue to avoid missing any attention event
from sink.

Change-Id: I36fd45efe1aa9437d5551002fc75a5f37d9cef79
Signed-off-by: Ajay Singh Parmar <aparmar@codeaurora.org>