msm: mdss: enable vsync event during continuous splash

Currently when continuous splash screen is enabled the vsync
event reports are delayed until first frame update. This blocks
boot animation. To fix this issue, after user space enables
vsync through vsync_ctrl, mdp driver will start sending vsync
events.

CRs-fixed: 567649
Change-Id: Ifb35e3479805e595d950ab7c38c26de201aa2d16
Signed-off-by: Ray Zhang <rayzhang@codeaurora.org>