mdss: mdp3: enhance vsync event report for mdp3

It is possible for client to register for vsync event before we
have commited a frame to display. It is in this case that we need
to fake vsync as it is not possible to get real vsync until we have
sent frame to be displayed.

Change-Id: Ied1821c847f22dbe703745fee1443c1dabdc4682
Signed-off-by: Terence Hampson <thampson@codeaurora.org>