fbdev: msm: Call vsync_handler only after ctl start

In hdmi-as-primary usecase with resolution change, the
vsync_handler can get called from overlay_on function before
the vsync handler is registered via ctl_start. Add a check
to avoid the null ptr access while accessing the vsync
handler function ptr in this case.

Change-Id: Ic47b10502939c4a4751c78da1f8fe28b3005cf3b
Signed-off-by: Nirmal Abraham <nabrah@codeaurora.org>