msm: wfd: Flush encoder after stopping VSG

VSG_STOP's completion is dependent on all buffers being returned back
to VSG. Hence, flush encoder earlier so that the core returns all input
buffers back.

Change-Id: I0cdef7e638f265b7bb9edc20623e608913823998
Signed-off-by: Deva Ramasubramanian <dramasub@codeaurora.org>