msm: mdp: Use a waitqueue for vsync notifications

Use a waitqueue instead of completions and make sure the vsync
show event always returns the timestamp except when it is
interrupted. This simplifies the vsync wait mechanism both here
and in userspace and avoids any unnecessary locking.

Change-Id: Ia3c2fce24e50d6b1e7fa1b36c668d5cc8678a1c6
Signed-off-by: Naseer Ahmed <naseer@codeaurora.org>