msm: mdss: avoid vsync_handler update in lp2 power state

vsync_handler added in lp2 (doze_suspend) power
state enables the vsync on hardware. That can lead
to unclocked register access because device can go
in pm_suspend in this power state. This change blocks
the vsync_handler processing in lp2 power state.

Change-Id: I4386baa6bc2f8303928edade79108b4983f66f42
Signed-off-by: Dhaval Patel <pdhaval@codeaurora.org>