msm: mdss: Change vsync dispatch mechanism

Notify the userspace when to read vsync instead of userspace
blocking on the vsync show event. Use a work queue to call
sysfs_notify to accomplish this.

Change-Id: I2d6ea6a6c4683cbd4c3ed9ca355546282e375194
Signed-off-by: Naseer Ahmed <naseer@codeaurora.org>