msm: mdss: hdmi: refactor event handler

HDMI driver handles many MDP events. Currently all the events
are handled in one function. Create different functions for
each event for a cleaner approach.

Change-Id: Ib48c02b2b89f7fdb347afd3a02d8b6b7c1c03e85
Signed-off-by: Ajay Singh Parmar <aparmar@codeaurora.org>