msm: camera: Fix memory leak with stats buffers

Buffer unprepare is not done while releasing stats buffers,
hence add support for buf unprepare ioctl

Change-Id: If4fa95c270363d1948b5924c2365785d1427e2f4
Signed-off-by: Lakshmi Narayana Kalavala <lkalaval@codeaurora.org>
diff --git a/include/media/msm_isp.h b/include/media/msm_isp.h
index 3df6ded..8de8c37 100644
--- a/include/media/msm_isp.h
+++ b/include/media/msm_isp.h
@@ -206,6 +206,7 @@
 #define VFE_CMD_STATS_REQBUF                            139
 #define VFE_CMD_STATS_ENQUEUEBUF                        140
 #define VFE_CMD_STATS_FLUSH_BUFQ                        141
+#define VFE_CMD_STATS_UNREGBUF                          142
 
 struct msm_isp_cmd {
 	int32_t  id;