msm: display: Do not commit in unset for external.

In case of external, commit is happening in unset itself which
is causing flickers. This patch fixes the issue.

Change-Id: Id55346be7a155108954e23bef236db80e7b65152
Signed-off-by: Vishnuvardhan Prodduturi <vproddut@codeaurora.org>
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>