hwc: Handle properly event data string from driver

Now that multiple events are handled by the vsync thread,
the data read from the driver needs to be null terminated.
Also memset vdata string to zero to ensure that data from a
previously read event isn't considered part of the current
event with a shorter data string.

Change-Id: Iff63e3a817c289ebca8a735ec2484c067b3ff3cf
1 file changed