msm: mdss: Do not time out for vsync reads

The userspace does a blocking read on the vsync node. However,
when the screen is idle the read returns after timeout waking up
the userspace thread. Remove the timeout so that the read blocks
until the vsync is updated.

Change-Id: Ieaa1c180d473256da14302875dfc0558d23123aa
Signed-off-by: Naseer Ahmed <naseer@codeaurora.org>