video: msm: wfd: Reset "stopped" state on vsg_start

In the sequence of vsg_start(), vsg_stop() and vsg_start(), the second
vsg_start() wouldn't generate buffers as the timer thread would
immediately stop due to to VSG still being in a stopped state.

Change-Id: I0b2d31c81d46e91b898a6210ad2c50d5c9200f1c
Signed-off-by: Deva Ramasubramanian <dramasub@codeaurora.org>