msm: mdss: avoid scheduling pp work during continuous updates

Ping pong work is only relevant when last frame update happens to go
into idle screen cases. If a commit is in progress there is no need to
schedule this work.

bug 30115868

CRs-Fixed: 1081525
Change-Id: I61956a4038f80f55d9507be87d7f7ee278ccf45e
Signed-off-by: Adrian Salido <salidoa@google.com>
(cherry picked from commit e49a573e0ece576ece60c087595ea12ae4191ae8)
Signed-off-by: Abhijit Kulkarni <kabhijit@codeaurora.org>