disp: msm: sde: hold vmlock only during transition in check phase

In the current code, vmlock is always acquired in check
phase even if there is no transition between vm's. This
might result in janks if vmlock is held concurrently by
other processes such as backlight update. This change
ensures that vmlock is held only if there is a valid
transition request between vm's in check phase.

Change-Id: I022f04c19ba04fdd5494580cc1436747620b9354
Signed-off-by: Yashwanth <yvulapu@codeaurora.org>