msm: wfd: Clear out stale flags for output buffers

The V4L2 framework doesn't appear to be clearing out vendor specific
flags between each QBUF call.  Hence clear out all flags that we set
explicitly in the driver.

Change-Id: Id1ec27c5971af0f691906738d0b5150e9f54b262
Signed-off-by: Deva Ramasubramanian <dramasub@codeaurora.org>