msm_vidc: venc: Release encoder buffers

Video encoder session cleanup was not done completely, this
change allows cleaning up of the scratch buffers and ensures
firmware releases output buffers before driver frees them.

Change-Id: I08e825376c81311c4235bbf7d6dd81de969bee73
Signed-off-by: Ashray Kulkarni <ashrayk@codeaurora.org>
Signed-off-by: Vinay Kalia <vkalia@codeaurora.org>