msm: vidc: Do not release capture buffers during flush

If capture buffers are freed before flush_done, then
VIDIOC_DQBUF will fail, since driver will not have the
capture buffers registered with the instance. This results
in video playback failing after port reconfig. This change
fixes the issue.

Change-Id: I5fe40c5dbd7977a8b78a1bb6480c00bb0c86e695
Signed-off-by: Arun Menon <avmenon@codeaurora.org>