msm: mdss: implement vsync ctrl and events

Allow users to enable vsync events to be sent as uevents to userspace
applications. This allows applications to listen for and align
composition to hardware vsync.

Change-Id: Ice480659f8c2adda3b5fe3f2d5bcfb9bf779933c
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>