msm: mdss: handle vsync properly during dynamic resolution switch

During resolution switch, the vsync is disabled just before the switch
as part of ctrl stop, but it is not enabled back after the switch. This
change keeps track of vsync before the switch and handles it
appropriately after the switch.

Change-Id: I3eed93d7f7635217bac3c19c61f68154cacef56e
Signed-off-by: Harsh Sahu <hsahu@codeaurora.org>